Drupal Schema API and Update Manager: rollback support for applied hook_update_N schema changes

Answer

Drupal core does not provide a supported way to roll back a specific hook_update_N schema change; updates are applied forward‑only and recorded as a monotonically increasing module version.

Confirmed facts

• The Schema API defines only hook_update_N for forward migration; no hook_update_N_down contract exists.
• The Update Manager treats each schema change as immutable; reversal is not part of its supported workflow.
• Configuration synchronization can revert config but cannot revert schema changes.
• Transaction or DDL rollback depends on the database driver and only applies to the current update transaction.

Practical steps for this case

1. Verify the module’s declared schema version in .info.yml and confirm the version stored in the system table after the update.
2. Inspect the module code for any hook_update_N_down() implementation; if none exists, a direct rollback is not possible.
3. Back up the database before attempting any change.
4. If you need to undo the change, reinstall the module (uninstall then reinstall) or restore the database from a backup taken before the update.
5. Optionally, re‑run update.php to re‑apply the update, then manually execute any down logic the module provides.

Missing diagnostic

Please confirm whether the module defines a hook_update_N_down() method or if the database engine supports transactional DDL for the update, as this may affect the recommended approach.