diff --git a/backend/main.py b/backend/main.py
index 4d9cc1e..2d414f3 100644
--- a/backend/main.py
+++ b/backend/main.py
@@ -355,8 +355,11 @@ async def nrl_detail_page(
     ).first()
 
     assigned_unit = None
+    assigned_modem = None
     if assignment:
         assigned_unit = db.query(RosterUnit).filter_by(id=assignment.unit_id).first()
+        if assigned_unit and assigned_unit.deployed_with_modem_id:
+            assigned_modem = db.query(RosterUnit).filter_by(id=assigned_unit.deployed_with_modem_id).first()
 
     # Get session count
     session_count = db.query(MonitoringSession).filter_by(location_id=location_id).count()
@@ -393,6 +396,7 @@ async def nrl_detail_page(
         "location": location,
         "assignment": assignment,
         "assigned_unit": assigned_unit,
+        "assigned_modem": assigned_modem,
         "session_count": session_count,
         "file_count": file_count,
         "active_session": active_session,
diff --git a/backend/routers/project_locations.py b/backend/routers/project_locations.py
index 44fcdd5..e3dff52 100644
--- a/backend/routers/project_locations.py
+++ b/backend/routers/project_locations.py
@@ -353,18 +353,18 @@ async def assign_unit_to_location(
             detail=f"Unit type '{unit.device_type}' does not match location type '{location.location_type}'",
         )
 
-    # Check if location already has an active assignment
+    # Check if location already has an active assignment (active = assigned_until IS NULL)
     existing_assignment = db.query(UnitAssignment).filter(
         and_(
             UnitAssignment.location_id == location_id,
-            UnitAssignment.status == "active",
+            UnitAssignment.assigned_until == None,
         )
     ).first()
 
     if existing_assignment:
         raise HTTPException(
             status_code=400,
-            detail=f"Location already has an active unit assignment ({existing_assignment.unit_id}). Unassign first.",
+            detail=f"Location already has an active unit assignment ({existing_assignment.unit_id}). Use swap to replace it.",
         )
 
     # Create new assignment
@@ -433,10 +433,120 @@ async def unassign_unit(
     return {"success": True, "message": "Unit unassigned successfully"}
 
 
+@router.post("/locations/{location_id}/swap")
+async def swap_unit_on_location(
+    project_id: str,
+    location_id: str,
+    request: Request,
+    db: Session = Depends(get_db),
+):
+    """
+    Swap the unit assigned to a vibration monitoring location.
+    Ends the current active assignment (if any), creates a new one,
+    and optionally updates modem pairing on the seismograph.
+    Works for first-time assignments too (no current assignment = just create).
+    """
+    location = db.query(MonitoringLocation).filter_by(
+        id=location_id,
+        project_id=project_id,
+    ).first()
+    if not location:
+        raise HTTPException(status_code=404, detail="Location not found")
+
+    form_data = await request.form()
+    unit_id = form_data.get("unit_id")
+    modem_id = form_data.get("modem_id") or None
+    notes = form_data.get("notes") or None
+
+    if not unit_id:
+        raise HTTPException(status_code=400, detail="unit_id is required")
+
+    # Validate new unit
+    unit = db.query(RosterUnit).filter_by(id=unit_id).first()
+    if not unit:
+        raise HTTPException(status_code=404, detail="Unit not found")
+
+    expected_device_type = "slm" if location.location_type == "sound" else "seismograph"
+    if unit.device_type != expected_device_type:
+        raise HTTPException(
+            status_code=400,
+            detail=f"Unit type '{unit.device_type}' does not match location type '{location.location_type}'",
+        )
+
+    # End current active assignment if one exists (active = assigned_until IS NULL)
+    current = db.query(UnitAssignment).filter(
+        and_(
+            UnitAssignment.location_id == location_id,
+            UnitAssignment.assigned_until == None,
+        )
+    ).first()
+    if current:
+        current.assigned_until = datetime.utcnow()
+        current.status = "completed"
+
+    # Create new assignment
+    new_assignment = UnitAssignment(
+        id=str(uuid.uuid4()),
+        unit_id=unit_id,
+        location_id=location_id,
+        project_id=project_id,
+        device_type=unit.device_type,
+        assigned_until=None,
+        status="active",
+        notes=notes,
+    )
+    db.add(new_assignment)
+
+    # Update modem pairing on the seismograph if modem provided
+    if modem_id:
+        modem = db.query(RosterUnit).filter_by(id=modem_id, device_type="modem").first()
+        if not modem:
+            raise HTTPException(status_code=404, detail=f"Modem '{modem_id}' not found")
+        unit.deployed_with_modem_id = modem_id
+        modem.deployed_with_unit_id = unit_id
+    else:
+        # Clear modem pairing if not provided
+        unit.deployed_with_modem_id = None
+
+    db.commit()
+
+    return JSONResponse({
+        "success": True,
+        "assignment_id": new_assignment.id,
+        "message": f"Unit '{unit_id}' assigned to '{location.name}'" + (f" with modem '{modem_id}'" if modem_id else ""),
+    })
+
+
 # ============================================================================
 # Available Units for Assignment
 # ============================================================================
 
+@router.get("/available-modems", response_class=JSONResponse)
+async def get_available_modems(
+    project_id: str,
+    db: Session = Depends(get_db),
+):
+    """
+    Get all deployed, non-retired modems for the modem assignment dropdown.
+    """
+    modems = db.query(RosterUnit).filter(
+        and_(
+            RosterUnit.device_type == "modem",
+            RosterUnit.deployed == True,
+            RosterUnit.retired == False,
+        )
+    ).order_by(RosterUnit.id).all()
+
+    return [
+        {
+            "id": m.id,
+            "hardware_model": m.hardware_model,
+            "ip_address": m.ip_address,
+        }
+        for m in modems
+    ]
+
+
 @router.get("/available-units", response_class=JSONResponse)
 async def get_available_units(
     project_id: str,
diff --git a/templates/projects/detail.html b/templates/projects/detail.html
index 5a4a8cf..d559017 100644
--- a/templates/projects/detail.html
+++ b/templates/projects/detail.html
@@ -838,6 +838,7 @@ async function loadProjectDetails() {
 
         // Update tab labels and visibility based on project type
         const isSoundProject = projectTypeId === 'sound_monitoring';
+        const isVibrationProject = projectTypeId === 'vibration_monitoring';
         if (isSoundProject) {
             document.getElementById('locations-tab-label').textContent = 'NRLs';
             document.getElementById('locations-header').textContent = 'Noise Recording Locations';
@@ -848,9 +849,9 @@ async function loadProjectDetails() {
         const isRemote = mode === 'remote';
         document.getElementById('sessions-tab-btn').classList.toggle('hidden', !isSoundProject);
         document.getElementById('data-tab-btn').classList.toggle('hidden', !isSoundProject);
-        // Schedules and Assigned Units are remote-only (manual projects collect data by hand)
-        document.getElementById('schedules-tab-btn')?.classList.toggle('hidden', isSoundProject && !isRemote);
-        document.getElementById('units-tab-btn')?.classList.toggle('hidden', isSoundProject && !isRemote);
+        // Schedules and Assigned Units: hidden for vibration; for sound, only show if remote
+        document.getElementById('schedules-tab-btn')?.classList.toggle('hidden', isVibrationProject || (isSoundProject && !isRemote));
+        document.getElementById('units-tab-btn')?.classList.toggle('hidden', isVibrationProject || (isSoundProject && !isRemote));
         // FTP browser within Data Files tab
         document.getElementById('ftp-browser')?.classList.toggle('hidden', !isRemote);
 
diff --git a/templates/vibration_location_detail.html b/templates/vibration_location_detail.html
index acb41dd..6afc9d8 100644
--- a/templates/vibration_location_detail.html
+++ b/templates/vibration_location_detail.html
@@ -37,7 +37,7 @@
                 {{ location.name }}
             </h1>
             <p class="text-gray-600 dark:text-gray-400 mt-1">
-                Monitoring Location • {{ project.name }}
+                Monitoring Location &bull; {{ project.name }}
             </p>
         </div>
         <div class="flex gap-2">
@@ -116,20 +116,36 @@
                 <h2 class="text-xl font-semibold text-gray-900 dark:text-white mb-4">Unit Assignment</h2>
                 {% if assigned_unit %}
                 <div class="space-y-4">
+                    <!-- Seismograph row -->
                     <div>
-                        <div class="text-sm text-gray-600 dark:text-gray-400">Assigned Unit</div>
+                        <div class="text-sm text-gray-600 dark:text-gray-400">Seismograph</div>
                         <div class="text-lg font-medium text-gray-900 dark:text-white">
                             <a href="/unit/{{ assigned_unit.id }}" class="text-seismo-orange hover:text-seismo-navy">
                                 {{ assigned_unit.id }}
                             </a>
                         </div>
+                        {% if assigned_unit.unit_type %}
+                        <div class="text-xs text-gray-500 dark:text-gray-400 mt-0.5">{{ assigned_unit.unit_type }}</div>
+                        {% endif %}
                     </div>
-                    {% if assigned_unit.device_type %}
+                    <!-- Modem row -->
                     <div>
-                        <div class="text-sm text-gray-600 dark:text-gray-400">Device Type</div>
-                        <div class="text-gray-900 dark:text-white">{{ assigned_unit.device_type|capitalize }}</div>
+                        <div class="text-sm text-gray-600 dark:text-gray-400">Modem</div>
+                        {% if assigned_modem %}
+                        <div class="text-lg font-medium text-gray-900 dark:text-white">
+                            <a href="/unit/{{ assigned_modem.id }}" class="text-seismo-orange hover:text-seismo-navy">
+                                {{ assigned_modem.id }}
+                            </a>
+                        </div>
+                        {% if assigned_modem.hardware_model or assigned_modem.ip_address %}
+                        <div class="text-xs text-gray-500 dark:text-gray-400 mt-0.5">
+                            {{ assigned_modem.hardware_model or '' }}{% if assigned_modem.hardware_model and assigned_modem.ip_address %} &bull; {% endif %}{{ assigned_modem.ip_address or '' }}
+                        </div>
+                        {% endif %}
+                        {% else %}
+                        <div class="text-sm text-gray-400 dark:text-gray-500 italic">No modem paired</div>
+                        {% endif %}
                     </div>
-                    {% endif %}
                     {% if assignment %}
                     <div>
                         <div class="text-sm text-gray-600 dark:text-gray-400">Assigned Since</div>
@@ -142,10 +158,14 @@
                     </div>
                     {% endif %}
                     {% endif %}
-                    <div class="pt-2">
+                    <div class="pt-2 flex gap-2 flex-wrap">
+                        <button onclick="openSwapModal()"
+                                class="px-4 py-2 bg-seismo-orange text-white rounded-lg hover:bg-seismo-navy transition-colors text-sm">
+                            Swap Unit / Modem
+                        </button>
                         <button onclick="unassignUnit('{{ assignment.id }}')"
-                                class="px-4 py-2 bg-amber-100 text-amber-800 dark:bg-amber-900/30 dark:text-amber-300 rounded-lg hover:bg-amber-200 dark:hover:bg-amber-900/50 transition-colors">
-                            Unassign Unit
+                                class="px-4 py-2 bg-amber-100 text-amber-800 dark:bg-amber-900/30 dark:text-amber-300 rounded-lg hover:bg-amber-200 dark:hover:bg-amber-900/50 transition-colors text-sm">
+                            Unassign
                         </button>
                     </div>
                 </div>
@@ -155,7 +175,7 @@
                         <path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M20 13V6a2 2 0 00-2-2H6a2 2 0 00-2 2v7m16 0v5a2 2 0 01-2 2H6a2 2 0 01-2-2v-5m16 0h-2.586a1 1 0 00-.707.293l-2.414 2.414a1 1 0 01-.707.293h-3.172a1 1 0 01-.707-.293l-2.414-2.414A1 1 0 006.586 13H4"></path>
                     </svg>
                     <p class="text-gray-500 dark:text-gray-400 mb-4">No unit currently assigned</p>
-                    <button onclick="openAssignModal()"
+                    <button onclick="openSwapModal()"
                             class="px-4 py-2 bg-seismo-orange text-white rounded-lg hover:bg-seismo-navy transition-colors">
                         Assign a Unit
                     </button>
@@ -214,47 +234,55 @@
     </div>
 </div>
 
-<!-- Assign Unit Modal -->
-<div id="assign-modal" class="hidden fixed inset-0 bg-black bg-opacity-50 z-50 flex items-center justify-center">
+<!-- Assign / Swap Modal -->
+<div id="swap-modal" class="hidden fixed inset-0 bg-black bg-opacity-50 z-50 flex items-center justify-center">
     <div class="bg-white dark:bg-slate-800 rounded-xl shadow-2xl w-full max-w-2xl max-h-[90vh] overflow-y-auto m-4">
         <div class="p-6 border-b border-gray-200 dark:border-gray-700 flex items-center justify-between">
             <div>
-                <h2 class="text-2xl font-bold text-gray-900 dark:text-white">Assign Unit</h2>
-                <p class="text-gray-600 dark:text-gray-400 mt-1">Attach a seismograph to this location</p>
+                <h2 id="swap-modal-title" class="text-2xl font-bold text-gray-900 dark:text-white">Assign Unit</h2>
+                <p class="text-gray-600 dark:text-gray-400 mt-1">Select a seismograph and optionally a modem for this location</p>
             </div>
-            <button onclick="closeAssignModal()" class="text-gray-500 hover:text-gray-700 dark:text-gray-400 dark:hover:text-gray-200">
+            <button onclick="closeSwapModal()" class="text-gray-500 hover:text-gray-700 dark:text-gray-400 dark:hover:text-gray-200">
                 <svg class="w-6 h-6" fill="none" stroke="currentColor" viewBox="0 0 24 24">
                     <path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M6 18L18 6M6 6l12 12"></path>
                 </svg>
             </button>
         </div>
 
-        <form id="assign-form" class="p-6 space-y-4">
+        <form id="swap-form" class="p-6 space-y-4">
             <div>
-                <label class="block text-sm font-medium text-gray-700 dark:text-gray-300 mb-2">Available Units</label>
-                <select id="assign-unit-id" name="unit_id"
+                <label class="block text-sm font-medium text-gray-700 dark:text-gray-300 mb-2">Seismograph <span class="text-red-500">*</span></label>
+                <select id="swap-unit-id" name="unit_id"
                         class="w-full px-4 py-2 border border-gray-300 dark:border-gray-600 rounded-lg bg-white dark:bg-gray-700 text-gray-900 dark:text-white" required>
                     <option value="">Loading units...</option>
                 </select>
-                <p id="assign-empty" class="hidden text-xs text-gray-500 mt-2">No available seismographs for this project.</p>
+                <p id="swap-units-empty" class="hidden text-xs text-gray-500 mt-1">No available seismographs.</p>
+            </div>
+
+            <div>
+                <label class="block text-sm font-medium text-gray-700 dark:text-gray-300 mb-2">Modem <span class="text-xs text-gray-400">(optional)</span></label>
+                <select id="swap-modem-id" name="modem_id"
+                        class="w-full px-4 py-2 border border-gray-300 dark:border-gray-600 rounded-lg bg-white dark:bg-gray-700 text-gray-900 dark:text-white">
+                    <option value="">No modem</option>
+                </select>
             </div>
 
             <div>
                 <label class="block text-sm font-medium text-gray-700 dark:text-gray-300 mb-2">Notes</label>
-                <textarea id="assign-notes" name="notes" rows="2"
+                <textarea id="swap-notes" name="notes" rows="2"
                           class="w-full px-4 py-2 border border-gray-300 dark:border-gray-600 rounded-lg bg-white dark:bg-gray-700 text-gray-900 dark:text-white"></textarea>
             </div>
 
-            <div id="assign-error" class="hidden text-sm text-red-600"></div>
+            <div id="swap-error" class="hidden text-sm text-red-600"></div>
 
             <div class="flex justify-end gap-3 pt-2">
-                <button type="button" onclick="closeAssignModal()"
+                <button type="button" onclick="closeSwapModal()"
                         class="px-6 py-2 border border-gray-300 dark:border-gray-600 rounded-lg text-gray-700 dark:text-gray-300 hover:bg-gray-50 dark:hover:bg-gray-700">
                     Cancel
                 </button>
-                <button type="submit"
+                <button type="submit" id="swap-submit-btn"
                         class="px-6 py-2 bg-seismo-orange hover:bg-seismo-navy text-white rounded-lg font-medium">
-                    Assign Unit
+                    Assign
                 </button>
             </div>
         </form>
@@ -264,6 +292,7 @@
 <script>
 const projectId = "{{ project_id }}";
 const locationId = "{{ location_id }}";
+const hasAssignment = {{ 'true' if assigned_unit else 'false' }};
 
 // Tab switching
 function switchTab(tabName) {
@@ -314,60 +343,89 @@ document.getElementById('location-settings-form').addEventListener('submit', asy
     }
 });
 
-// Assign modal
-function openAssignModal() {
-    document.getElementById('assign-modal').classList.remove('hidden');
-    loadAvailableUnits();
+// Swap / Assign modal
+async function openSwapModal() {
+    document.getElementById('swap-modal').classList.remove('hidden');
+    document.getElementById('swap-modal-title').textContent = hasAssignment ? 'Swap Unit / Modem' : 'Assign Unit';
+    document.getElementById('swap-submit-btn').textContent = hasAssignment ? 'Swap' : 'Assign';
+    document.getElementById('swap-error').classList.add('hidden');
+    document.getElementById('swap-notes').value = '';
+    await Promise.all([loadSwapUnits(), loadSwapModems()]);
 }
 
-function closeAssignModal() {
-    document.getElementById('assign-modal').classList.add('hidden');
+function closeSwapModal() {
+    document.getElementById('swap-modal').classList.add('hidden');
 }
 
-async function loadAvailableUnits() {
+async function loadSwapUnits() {
     try {
         const response = await fetch(`/api/projects/${projectId}/available-units?location_type=vibration`);
-        if (!response.ok) throw new Error('Failed to load available units');
+        if (!response.ok) throw new Error('Failed to load units');
         const data = await response.json();
-        const select = document.getElementById('assign-unit-id');
-        select.innerHTML = '<option value="">Select a unit</option>';
+        const select = document.getElementById('swap-unit-id');
+        select.innerHTML = '<option value="">Select a seismograph</option>';
 
         if (!data.length) {
-            document.getElementById('assign-empty').classList.remove('hidden');
-            return;
+            document.getElementById('swap-units-empty').classList.remove('hidden');
+        } else {
+            document.getElementById('swap-units-empty').classList.add('hidden');
         }
 
         data.forEach(unit => {
             const option = document.createElement('option');
             option.value = unit.id;
-            option.textContent = `${unit.id} • ${unit.model || unit.device_type}`;
+            option.textContent = unit.id + (unit.model ? ` \u2022 ${unit.model}` : '') + (unit.location ? ` \u2014 ${unit.location}` : '');
             select.appendChild(option);
         });
     } catch (err) {
-        const errorEl = document.getElementById('assign-error');
-        errorEl.textContent = err.message || 'Failed to load units.';
-        errorEl.classList.remove('hidden');
+        document.getElementById('swap-error').textContent = 'Failed to load seismographs.';
+        document.getElementById('swap-error').classList.remove('hidden');
     }
 }
 
-document.getElementById('assign-form').addEventListener('submit', async function(e) {
+async function loadSwapModems() {
+    try {
+        const response = await fetch(`/api/projects/${projectId}/available-modems`);
+        if (!response.ok) throw new Error('Failed to load modems');
+        const data = await response.json();
+        const select = document.getElementById('swap-modem-id');
+        select.innerHTML = '<option value="">No modem</option>';
+
+        data.forEach(modem => {
+            const option = document.createElement('option');
+            option.value = modem.id;
+            let label = modem.id;
+            if (modem.hardware_model) label += ` \u2022 ${modem.hardware_model}`;
+            if (modem.ip_address) label += ` \u2014 ${modem.ip_address}`;
+            option.textContent = label;
+            select.appendChild(option);
+        });
+    } catch (err) {
+        // Modem list failure is non-fatal — just leave blank
+        console.warn('Failed to load modems:', err);
+    }
+}
+
+document.getElementById('swap-form').addEventListener('submit', async function(e) {
     e.preventDefault();
 
-    const unitId = document.getElementById('assign-unit-id').value;
-    const notes = document.getElementById('assign-notes').value.trim();
+    const unitId = document.getElementById('swap-unit-id').value;
+    const modemId = document.getElementById('swap-modem-id').value;
+    const notes = document.getElementById('swap-notes').value.trim();
 
     if (!unitId) {
-        document.getElementById('assign-error').textContent = 'Select a unit to assign.';
-        document.getElementById('assign-error').classList.remove('hidden');
+        document.getElementById('swap-error').textContent = 'Please select a seismograph.';
+        document.getElementById('swap-error').classList.remove('hidden');
         return;
     }
 
     try {
         const formData = new FormData();
         formData.append('unit_id', unitId);
-        formData.append('notes', notes);
+        if (modemId) formData.append('modem_id', modemId);
+        if (notes) formData.append('notes', notes);
 
-        const response = await fetch(`/api/projects/${projectId}/locations/${locationId}/assign`, {
+        const response = await fetch(`/api/projects/${projectId}/locations/${locationId}/swap`, {
             method: 'POST',
             body: formData
         });
@@ -379,9 +437,8 @@ document.getElementById('assign-form').addEventListener('submit', async function
 
         window.location.reload();
     } catch (err) {
-        const errorEl = document.getElementById('assign-error');
-        errorEl.textContent = err.message || 'Failed to assign unit.';
-        errorEl.classList.remove('hidden');
+        document.getElementById('swap-error').textContent = err.message || 'Failed to assign unit.';
+        document.getElementById('swap-error').classList.remove('hidden');
     }
 });
 
@@ -405,11 +462,11 @@ async function unassignUnit(assignmentId) {
 }
 
 document.addEventListener('keydown', function(e) {
-    if (e.key === 'Escape') closeAssignModal();
+    if (e.key === 'Escape') closeSwapModal();
 });
 
-document.getElementById('assign-modal')?.addEventListener('click', function(e) {
-    if (e.target === this) closeAssignModal();
+document.getElementById('swap-modal')?.addEventListener('click', function(e) {
+    if (e.target === this) closeSwapModal();
 });
 </script>
 {% endblock %}