diff --git a/tests/update.bats b/tests/update.bats
index 4338cba..41424ea 100644
--- a/tests/update.bats
+++ b/tests/update.bats
@@ -166,3 +166,20 @@ ran() {
   [ "$status" -eq 1 ]
   [ ! -e "$RAN" ]
 }
+
+@test "an update from before 2026-10-19 that copies this one over itself hands over to it" {
+  # 013acac's update: sudo cp overwrites the file bash is still reading, and
+  # bash reads on in the new one from where it was
+  [ "$(/usr/bin/git -C "$REPO" rev-parse --is-shallow-repository)" = false ] \
+    || skip "needs update's history"
+  /usr/bin/git -C "$REPO" show 013acac:update > "$HOME/.local/bin/update"
+  chmod +x "$HOME/.local/bin/update"
+  stub sudo '[ "$1" = cp ] && exec "$@"; exit 0'
+  hook 10-a 'echo a >> "$RAN"'
+
+  run "$HOME/.local/bin/update"
+  [ "$status" -eq 0 ]
+  [[ "$output" != *"command not found"* ]]
+  [[ "$output" == *"Done."* ]]
+  ran a
+}
diff --git a/update b/update
index 402d87f..edea6c9 100755
--- a/update
+++ b/update
@@ -1,5 +1,29 @@
 #!/bin/bash
 
+# Updates from before 2026-10-19 (013acac's) copy this file over themselves
+# while bash is still reading them, then read on from byte 1484 of the new copy.
+# The exec line below starts at exactly that byte, so they run this update
+# afresh; the rest of the block pads it into place, and in a normal run the
+# block is a here-document nothing reads. Keep the exec line at byte 1484:
+# tests/update.bats checks it.
+: << 'FROM_OLD_UPDATE'
+This block is padding for old updates. This block is padding for old updates.
+This block is padding for old updates. This block is padding for old updates.
+This block is padding for old updates. This block is padding for old updates.
+This block is padding for old updates. This block is padding for old updates.
+This block is padding for old updates. This block is padding for old updates.
+This block is padding for old updates. This block is padding for old updates.
+This block is padding for old updates. This block is padding for old updates.
+This block is padding for old updates. This block is padding for old updates.
+This block is padding for old updates. This block is padding for old updates.
+This block is padding for old updates. This block is padding for old updates.
+This block is padding for old updates. This block is padding for old updates.
+This block is padding for old updates. This block is padding for old updates.
+This block is padding for old updates. This block is padding for old updates.
+...................
+exec ~/.local/bin/update
+FROM_OLD_UPDATE
+
 # The whole script is wrapped in { ...; exit; } so bash parses all of it before
 # running anything. That way, replacing this file mid-run (e.g. by git pull and
 # the copy below) can't make the running copy read a mix of old and new text.