Can a failed Bitbucket Server or Data Center upgrade be resumed, or is a full pre-upgrade restore required?

I need to decide the safe recovery path after a Bitbucket upgrade fails before completing the upgrade wizard. The goal is to avoid starting the application against a partially migrated database or mixed-version cluster nodes.

Bitbucket does not automatically roll back a failed upgrade, and database schema changes are not officially reversible. The upgrade log may indicate where the failure occurred, but it is unclear whether the wizard can be safely resumed on the existing BITBUCKET_HOME and database, or whether administrators must always restore the home directory and database dump from a pre-upgrade backup and restart the previous version. This distinction matters for single-node Server instances and multi-node Data Center clusters, where every node must run the same version.

Assumptions: Bitbucket Server or Data Center, a pre-upgrade backup exists, and the failure occurred during the upgrade wizard rather than after startup. Version alignment between binaries, home directory, and database must be confirmed before any restart.

Can the upgrade wizard resume after a failure without restoring from backup? Does resuming risk database schema inconsistency or unsupported mixed-version behavior? When is a full pre-upgrade restore the only supported option?