Index: deploy/MULTI-INSTANCE.md
===================================================================
--- deploy/MULTI-INSTANCE.md	(revision fa33214eb34752fafee25fc95942f470701006f8)
+++ deploy/MULTI-INSTANCE.md	(revision 12bed59eaefe12a4ed9c2c3cc267cb448ae42b43)
@@ -84,11 +84,21 @@
 because the port comes from the same `.env`.
 
-**Rolling back.** The old `klonkt.service` is disabled and masked, not deleted.
-Masked because `disable` alone does not stop `systemctl restart klonkt` from
-starting it again, and a resurrected unit no longer finds its `.env` (that moved
-with the data): it would fall back to the defaults and write a fresh empty
-database into the checkout. To go back, move the data into `/opt/klonkt/storage`,
-restore the relative paths in `.env`, then `systemctl unmask klonkt` and
-`systemctl enable --now klonkt`.
+**The old unit is moved aside, not deleted.** It ends up next to its old place
+as `klonkt.service.retired-<timestamp>`. Stopping and disabling is not enough:
+`systemctl restart klonkt` starts a disabled unit anyway, which is exactly what
+an updater generated before the split does. A resurrected `klonkt.service` no
+longer finds its `.env` (that moved with the data), falls back to the built-in
+defaults, and writes a fresh empty database into the checkout. Masking does not
+work here either, because the unit file sits in `/etc/systemd/system` and
+`systemctl mask` refuses while a real file is there.
+
+**Rolling back.** Move the data into `/opt/klonkt/storage`, restore the relative
+paths in `.env`, move the retired unit file back to
+`/etc/systemd/system/klonkt.service`, then:
+
+```bash
+sudo systemctl daemon-reload
+sudo systemctl enable --now klonkt
+```
 
 ## Adding an instance
Index: scripts/klonkt-migrate-data.sh
===================================================================
--- scripts/klonkt-migrate-data.sh	(revision fa33214eb34752fafee25fc95942f470701006f8)
+++ scripts/klonkt-migrate-data.sh	(revision 12bed59eaefe12a4ed9c2c3cc267cb448ae42b43)
@@ -128,16 +128,31 @@
 run "systemctl daemon-reload"
 
+step "Retiring $OLD_UNIT"
+# Stopping and disabling is NOT enough: `systemctl restart klonkt` starts a
+# disabled unit anyway, and that is exactly what an updater generated before
+# the split does. A resurrected klonkt.service no longer finds its .env (that
+# moved with the data), falls back to the built-in defaults, and writes a
+# FRESH EMPTY database into the checkout.
+#
+# Masking does not help either: the unit file lives in /etc/systemd/system,
+# the highest-priority directory, and `systemctl mask` refuses when a real
+# file is already there ("File ... already exists"). Verified, not assumed.
+#
+# So the file is moved aside. systemd then no longer knows the unit at all and
+# any restart fails loudly with "Unit klonkt.service not found". The file is
+# kept next to its old place, timestamped, so a rollback is a move back.
+if [ -f "/etc/systemd/system/$OLD_UNIT" ]; then
+  run "systemctl stop $OLD_UNIT 2>/dev/null || true"
+  run "systemctl disable $OLD_UNIT 2>/dev/null || true"
+  RETIRED="/etc/systemd/system/${OLD_UNIT}.retired-$(date +%Y%m%d%H%M%S)"
+  run "mv '/etc/systemd/system/$OLD_UNIT' '$RETIRED'"
+  run "systemctl daemon-reload"
+  say "stopped, disabled and moved aside → $RETIRED"
+  say "roll back by moving that file back and running: systemctl daemon-reload"
+else
+  say "no $OLD_UNIT unit file to retire"
+fi
+
 step "Switching to klonkt@$SLUG"
-if systemctl list-unit-files "$OLD_UNIT" >/dev/null 2>&1; then
-  run "systemctl disable --now $OLD_UNIT"
-  # Disable only removes the autostart link: `systemctl restart klonkt` would
-  # still START it. That is not theoretical — an updater generated before the
-  # split does exactly that, and the resurrected unit finds no .env (it moved
-  # with the data), falls back to the built-in defaults and creates a FRESH
-  # EMPTY database in the checkout. Masking makes any such call fail loudly.
-  # Reversible: systemctl unmask klonkt.
-  run "systemctl mask $OLD_UNIT"
-  say "disabled and masked $OLD_UNIT (unmask to roll back)"
-fi
 run "systemctl enable --now 'klonkt@$SLUG'"
 
Index: scripts/klonkt-refresh-updater.sh
===================================================================
--- scripts/klonkt-refresh-updater.sh	(revision fa33214eb34752fafee25fc95942f470701006f8)
+++ scripts/klonkt-refresh-updater.sh	(revision 12bed59eaefe12a4ed9c2c3cc267cb448ae42b43)
@@ -59,16 +59,23 @@
 echo "klonkt-update rewritten: branch ${BRANCH}, code ${KLONKT_DIR}, instances under ${DATA_ROOT}"
 
-# On a split install the old single unit must not be startable. `disable` alone
-# does not stop `systemctl restart klonkt` from starting it, and a resurrected
-# klonkt.service has no .env (it moved with the data): it falls back to the
-# defaults and writes a fresh empty database into the checkout.
+# On a split install the old single unit must not be startable at all.
+# `disable` is not enough (restart starts a disabled unit anyway) and `mask`
+# refuses while the real file sits in /etc/systemd/system, the highest-priority
+# directory. Moving the file aside is what actually works: systemd stops
+# knowing the unit, so any restart fails loudly instead of quietly starting a
+# second process that writes an empty database into the checkout.
 SPLIT=0
 for d in "${DATA_ROOT}"/*/; do [ -f "$d/.env" ] && SPLIT=1 && break; done
-if [ "$SPLIT" = 1 ] && systemctl list-unit-files klonkt.service >/dev/null 2>&1; then
-  if ! systemctl is-enabled klonkt.service 2>/dev/null | grep -q masked; then
-    systemctl stop klonkt.service 2>/dev/null || true
-    systemctl disable klonkt.service 2>/dev/null || true
-    systemctl mask klonkt.service
-    echo "retired klonkt.service: stopped, disabled and masked (unmask to roll back)"
+if [ "$SPLIT" = 1 ] && [ -f /etc/systemd/system/klonkt.service ]; then
+  systemctl stop klonkt.service 2>/dev/null || true
+  systemctl disable klonkt.service 2>/dev/null || true
+  RETIRED="/etc/systemd/system/klonkt.service.retired-$(date +%Y%m%d%H%M%S)"
+  if mv /etc/systemd/system/klonkt.service "$RETIRED"; then
+    systemctl daemon-reload
+    echo "retired klonkt.service → $RETIRED (move it back + daemon-reload to roll back)"
+  else
+    echo "WARNING: could not move /etc/systemd/system/klonkt.service aside."
+    echo "         Until you do, any 'systemctl restart klonkt' starts a second"
+    echo "         process that writes an empty database into ${KLONKT_DIR}."
   fi
 fi