diff --git a/SeaHaven.Services/Implementation/WorkOrderBoardUpdateService.cs b/SeaHaven.Services/Implementation/WorkOrderBoardUpdateService.cs
index edbc3c8..f2c5bfb 100644
--- a/SeaHaven.Services/Implementation/WorkOrderBoardUpdateService.cs
+++ b/SeaHaven.Services/Implementation/WorkOrderBoardUpdateService.cs
@@ -841,6 +841,10 @@ namespace SeaHaven.Services.Implementation
             DateOnly? previousTargetWeek,
             List<FieldChange> changes)
         {
+            // An unmoved schedule keeps Add-On, including the create-time hint on unscheduled WOs (SH-412).
+            if (workOrder.ScheduledDate == previousScheduledDate && workOrder.TargetWeek == previousTargetWeek)
+                return;
+
             var oldValue = workOrder.IsAddOn.ToString();
             var newIsAddOn = WorkOrderAddonCutoff.RecalculateIsAddOnOnReschedule(
                 workOrder.IsAddOn,
diff --git a/SeaHavenIndustries.Tests/WorkOrderBoardUpdateServiceTests.cs b/SeaHavenIndustries.Tests/WorkOrderBoardUpdateServiceTests.cs
index 821ab59..28ecab4 100644
--- a/SeaHavenIndustries.Tests/WorkOrderBoardUpdateServiceTests.cs
+++ b/SeaHavenIndustries.Tests/WorkOrderBoardUpdateServiceTests.cs
@@ -2303,6 +2303,40 @@ public class WorkOrderBoardUpdateServiceTests
         Assert.Empty(await context.WorkOrderAuditLogs.Where(a => a.FieldName == "IsAddOn").ToListAsync());
     }
 
+    [Theory]
+    [InlineData("true")]
+    [InlineData("false")]
+    public async Task PatchField_ScheduleWeekOnlyToggle_Unscheduled_KeepsAddOnHint(string weekOnly)
+    {
+        var (context, service) = CreateSut(new FixedTimeProvider());
+        var wo = new WorkOrder
+        {
+            Id = 1,
+            LifecycleStatus = LifecycleStatus.Pending,
+            WorkOrderType = WorkOrderType.PM,
+            ScheduleWeekOnly = weekOnly != "true",
+            CreatedDate = new DateTime(2026, 6, 1, 0, 0, 0, DateTimeKind.Utc),
+            IsAddOn = true,
+            RowVersion = new byte[] { 1, 0, 0, 0, 0, 0, 0, 1 }
+        };
+        context.workOrders.Add(wo);
+        await context.SaveChangesAsync();
+
+        var result = await service.PatchFieldAsync(1, new WorkOrderBoardPatchRequestDto
+        {
+            Field = WorkOrderBoardFieldNames.ScheduleWeekOnly,
+            Value = weekOnly,
+            WorkOrderVersion = ToVersion(wo)
+        }, "actor-1");
+
+        Assert.True(result.IsAddOn);
+        var persisted = await context.workOrders.SingleAsync(w => w.Id == 1);
+        Assert.Null(persisted.ScheduledDate);
+        Assert.Null(persisted.TargetWeek);
+        Assert.True(persisted.IsAddOn);
+        Assert.Empty(await context.WorkOrderAuditLogs.Where(a => a.FieldName == "IsAddOn").ToListAsync());
+    }
+
     [Theory]
     [InlineData("2026-07-08", false, "2026-06-25", true)]  // into the current week, past its cutoff
     [InlineData("2026-06-24", true, "2026-07-01", false)]  // into next week, before its cutoff