diff --git a/src/daemon/systemd.ts b/src/daemon/systemd.ts
index 0a295436d..0e1dc5541 100644
--- a/src/daemon/systemd.ts
+++ b/src/daemon/systemd.ts
@@ -193,6 +193,18 @@ export async function installSystemdService({
 
   const unitPath = resolveSystemdUnitPath(env);
   await fs.mkdir(path.dirname(unitPath), { recursive: true });
+
+  // Preserve user customizations: back up existing unit file before overwriting.
+  let backedUp = false;
+  try {
+    await fs.access(unitPath);
+    const backupPath = `${unitPath}.bak`;
+    await fs.copyFile(unitPath, backupPath);
+    backedUp = true;
+  } catch {
+    // File does not exist yet — nothing to back up.
+  }
+
   const serviceDescription = resolveGatewayServiceDescription({ env, environment, description });
   const unit = buildSystemdUnit({
     description: serviceDescription,
@@ -227,6 +239,14 @@ export async function installSystemdService({
         label: "Installed systemd service",
         value: unitPath,
       },
+      ...(backedUp
+        ? [
+            {
+              label: "Previous unit backed up to",
+              value: `${unitPath}.bak`,
+            },
+          ]
+        : []),
     ],
     { leadingBlankLine: true },
   );