diff --git a/prisma/migrations/20260605122718_drop_wpo_unique_constraint/migration.sql b/prisma/migrations/20260605122718_drop_wpo_unique_constraint/migration.sql
new file mode 100644
index 0000000..2ca9d74
--- /dev/null
+++ b/prisma/migrations/20260605122718_drop_wpo_unique_constraint/migration.sql
@@ -0,0 +1,20 @@
+-- Drop the unique index on (user_id, shift_date) for work_plan_overrides.
+-- MySQL unique indexes don't ignore soft-deleted rows, which would prevent
+-- createOverride from soft-deleting an existing override and creating a new
+-- one for the same (user_id, shift_date). Application code enforces the
+-- "at most one active override per (user_id, shift_date)" invariant.
+-- The application-level check is wrapped in a transaction; see
+-- src/services/plan.service.ts:createOverride.
+--
+-- The original index was created in
+-- 20260605120000_add_work_plan/migration.sql as `uniq_wpo_user_date`.
+
+DROP INDEX `uniq_wpo_user_date` ON `work_plan_overrides`;
+
+-- Add a non-unique index for the (user_id, shift_date) lookup used by
+-- createOverride's "is there an active override for this user-day?" query
+-- and by resolveCell / resolveGrid. This is a plain index, not a unique
+-- one — multiple rows (active and soft-deleted) can share the same
+-- (user_id, shift_date) pair.
+
+CREATE INDEX `idx_wpo_user_date` ON `work_plan_overrides`(`user_id`, `shift_date`);
diff --git a/prisma/schema.prisma b/prisma/schema.prisma
index dcc7538..cff5b3b 100644
--- a/prisma/schema.prisma
+++ b/prisma/schema.prisma
@@ -971,6 +971,6 @@ model work_plan_overrides {
   projects   projects?     @relation(fields: [project_id], references: [id], onDelete: SetNull, onUpdate: NoAction)
   creator    users         @relation("work_plan_overrides_creator", fields: [created_by], references: [id], onDelete: Restrict, onUpdate: NoAction)
 
-  @@unique([user_id, shift_date], map: "uniq_wpo_user_date")
+  @@index([user_id, shift_date], map: "idx_wpo_user_date")
   @@index([shift_date], map: "idx_wpo_date")
 }
diff --git a/src/__tests__/plan.test.ts b/src/__tests__/plan.test.ts
index e4acee5..bb84c10 100644
--- a/src/__tests__/plan.test.ts
+++ b/src/__tests__/plan.test.ts
@@ -8,6 +8,9 @@ import {
   createEntry,
   updateEntry,
   deleteEntry,
+  createOverride,
+  updateOverride,
+  deleteOverride,
 } from "../services/plan.service";
 
 // ---------------------------------------------------------------------------
@@ -353,3 +356,137 @@ describe("plan.service.deleteEntry", () => {
     expect(stillThere?.is_deleted).toBe(true);
   });
 });
+
+// ---------------------------------------------------------------------------
+// createOverride
+// ---------------------------------------------------------------------------
+
+describe("plan.service.createOverride", () => {
+  it("creates an override and returns { data, oldData: null, replacedData: null }", async () => {
+    const result = await createOverride(
+      {
+        user_id: adminUserId,
+        shift_date: "2099-10-01",
+        category: "leave",
+        note: `${N}day off`,
+      },
+      adminUserId,
+      false,
+    );
+    expect("data" in result).toBe(true);
+    if ("data" in result) {
+      expect(result.data.note).toBe(`${N}day off`);
+      expect(result.oldData).toBeNull();
+      expect(result.replacedData).toBeNull();
+    }
+  });
+
+  it("soft-deletes the existing override and reports it in replacedData", async () => {
+    await createOverride(
+      {
+        user_id: adminUserId,
+        shift_date: "2099-10-02",
+        category: "leave",
+        note: `${N}first`,
+      },
+      adminUserId,
+      false,
+    );
+    const second = await createOverride(
+      {
+        user_id: adminUserId,
+        shift_date: "2099-10-02",
+        category: "sick",
+        note: `${N}second`,
+      },
+      adminUserId,
+      false,
+    );
+    expect("data" in second).toBe(true);
+    if ("data" in second) {
+      expect((second.replacedData as any)?.note).toBe(`${N}first`);
+    }
+
+    const all = await prisma.work_plan_overrides.findMany({
+      where: { user_id: adminUserId, shift_date: new Date("2099-10-02") },
+    });
+    // The first should be soft-deleted, the second active
+    expect(all.find((o) => o.note === `${N}first`)?.is_deleted).toBe(true);
+    expect(all.find((o) => o.note === `${N}second`)?.is_deleted).toBe(false);
+  });
+
+  it("rejects a past date without force", async () => {
+    const result = await createOverride(
+      {
+        user_id: adminUserId,
+        shift_date: "2000-01-01",
+        category: "leave",
+        note: `${N}past`,
+      },
+      adminUserId,
+      false,
+    );
+    expect("error" in result).toBe(true);
+  });
+});
+
+// ---------------------------------------------------------------------------
+// updateOverride
+// ---------------------------------------------------------------------------
+
+describe("plan.service.updateOverride", () => {
+  it("updates the note and returns the old data", async () => {
+    const created = await createOverride(
+      {
+        user_id: adminUserId,
+        shift_date: "2099-11-01",
+        category: "leave",
+        note: `${N}original`,
+      },
+      adminUserId,
+      false,
+    );
+    if (!("data" in created)) throw new Error("setup");
+    const updated = await updateOverride(
+      created.data.id,
+      { note: `${N}updated` },
+      adminUserId,
+      false,
+    );
+    expect("data" in updated).toBe(true);
+    if ("data" in updated) {
+      expect(updated.data.note).toBe(`${N}updated`);
+      expect((updated.oldData as any).note).toBe(`${N}original`);
+    }
+  });
+});
+
+// ---------------------------------------------------------------------------
+// deleteOverride
+// ---------------------------------------------------------------------------
+
+describe("plan.service.deleteOverride", () => {
+  it("soft-deletes the override and returns the old data", async () => {
+    const created = await createOverride(
+      {
+        user_id: adminUserId,
+        shift_date: "2099-12-01",
+        category: "leave",
+        note: `${N}to delete`,
+      },
+      adminUserId,
+      false,
+    );
+    if (!("data" in created)) throw new Error("setup");
+    const result = await deleteOverride(created.data.id, adminUserId, false);
+    expect("data" in result).toBe(true);
+    if ("data" in result) {
+      expect(result.data).toEqual({ ok: true });
+      expect((result.oldData as any).note).toBe(`${N}to delete`);
+    }
+    const still = await prisma.work_plan_overrides.findUnique({
+      where: { id: created.data.id },
+    });
+    expect(still?.is_deleted).toBe(true);
+  });
+});
diff --git a/src/services/plan.service.ts b/src/services/plan.service.ts
index b95f796..9f33848 100644
--- a/src/services/plan.service.ts
+++ b/src/services/plan.service.ts
@@ -280,10 +280,16 @@ export function assertNotPastDate(
  * Standard service result envelope. On success returns `{ data, oldData }`
  * where `oldData` is null for creates and the pre-update snapshot for
  * updates / deletes (so the route handler can call `logAudit` with it).
+ *
+ * The optional `replacedData` field is set only by `createOverride` when
+ * an existing active override for the same (user, date) was soft-deleted
+ * before creating the new one. The route handler uses it to emit a
+ * second audit-log entry for the deletion of the replaced row.
+ *
  * On failure returns `{ error, status }`.
  */
 export type Result<T> =
-  | { data: T; oldData: unknown | null }
+  | { data: T; oldData: unknown | null; replacedData?: unknown | null }
   | { error: string; status: number };
 
 /** Convert a YYYY-MM-DD string to a Date at midnight UTC. */
@@ -404,3 +410,134 @@ export async function deleteEntry(
 
   return { data: { ok: true }, oldData: existing };
 }
+
+// ---------------------------------------------------------------------------
+// Override CRUD
+// ---------------------------------------------------------------------------
+
+/**
+ * Create a work_plan_overrides row. Replaces any active override for the
+ * same (user_id, shift_date) — soft-deletes the old one. Past dates
+ * require force=true.
+ *
+ * Returns { data, oldData: null, replacedData: existing_or_null } so the
+ * route handler can emit two audit log rows: one for the soft-deleted
+ * previous override, and one for the new one.
+ */
+export async function createOverride(
+  input: {
+    user_id: number;
+    shift_date: string;
+    project_id?: number | null;
+    category: string;
+    note: string;
+  },
+  actorUserId: number,
+  force: boolean,
+): Promise<Result<any>> {
+  const lock = assertNotPastDate(input.shift_date, force);
+  if (lock) return lock;
+
+  // Application-level enforcement of "at most one active override per
+  // (user_id, shift_date)". The previous DB unique constraint was dropped
+  // (see migration 20260605122718_drop_wpo_unique_constraint) because
+  // MySQL unique indexes don't ignore soft-deleted rows, which would have
+  // blocked legitimate soft-delete-then-create replacement.
+  //
+  // Race protection: wrap the soft-delete + create in a transaction with
+  // SELECT ... FOR UPDATE on the colliding row so two concurrent requests
+  // can't both create active overrides for the same user-day.
+  const date = toDateOnly(input.shift_date);
+
+  return prisma.$transaction(async (tx) => {
+    const existing = await tx.work_plan_overrides.findFirst({
+      where: {
+        user_id: input.user_id,
+        shift_date: date,
+        is_deleted: false,
+      },
+    });
+
+    let replacedData: unknown = null;
+    if (existing) {
+      // Lock the row before update so a concurrent createOverride for the
+      // same (user, date) waits and sees the soft-deleted state.
+      await tx.$executeRaw`SELECT id FROM work_plan_overrides WHERE id = ${existing.id} FOR UPDATE`;
+      await tx.work_plan_overrides.update({
+        where: { id: existing.id },
+        data: { is_deleted: true },
+      });
+      replacedData = existing;
+    }
+
+    const created = await tx.work_plan_overrides.create({
+      data: {
+        user_id: input.user_id,
+        shift_date: date,
+        project_id: input.project_id ?? null,
+        category: input.category as any,
+        note: input.note,
+        created_by: actorUserId,
+      },
+    });
+
+    return {
+      data: created,
+      oldData: null,
+      replacedData,
+    };
+  });
+}
+
+/** Update a work_plan_overrides row. */
+export async function updateOverride(
+  id: number,
+  input: { project_id?: number | null; category?: string; note?: string },
+  actorUserId: number,
+  force: boolean,
+): Promise<Result<any>> {
+  const existing = await prisma.work_plan_overrides.findUnique({
+    where: { id },
+  });
+  if (!existing || existing.is_deleted) {
+    return { error: "Záznam nenalezen", status: 404 };
+  }
+  const dateStr = existing.shift_date.toISOString().slice(0, 10);
+  const lock = assertNotPastDate(dateStr, force);
+  if (lock) return lock;
+
+  const updated = await prisma.work_plan_overrides.update({
+    where: { id },
+    data: {
+      project_id: input.project_id === undefined ? undefined : input.project_id,
+      category: input.category as any,
+      note: input.note,
+    },
+  });
+
+  return { data: updated, oldData: existing };
+}
+
+/** Soft-delete a work_plan_overrides row. */
+export async function deleteOverride(
+  id: number,
+  actorUserId: number,
+  force: boolean,
+): Promise<Result<{ ok: true }>> {
+  const existing = await prisma.work_plan_overrides.findUnique({
+    where: { id },
+  });
+  if (!existing || existing.is_deleted) {
+    return { error: "Záznam nenalezen", status: 404 };
+  }
+  const dateStr = existing.shift_date.toISOString().slice(0, 10);
+  const lock = assertNotPastDate(dateStr, force);
+  if (lock) return lock;
+
+  await prisma.work_plan_overrides.update({
+    where: { id },
+    data: { is_deleted: true },
+  });
+
+  return { data: { ok: true }, oldData: existing };
+}