Treat the release as a data migration
Jellyfin’s 12.0 notes require an eligible preceding server version and a full backup before upgrading. They also identify a specific username problem: names differing only by capitalisation cannot coexist under the new case-insensitive rule and can make migration fail. Check those local accounts before starting the new server rather than assuming a package installation alone completes the upgrade.
Understand the post-upgrade library task
The release notes require a full scan after upgrading because automatically grouped versions are cleared and rebuilt from files. They warn that the first scan can take longer and that some items may appear newly added. That documented state is different from immediately concluding the underlying media was deleted. The notes separately address old plugins and clients, so their compatibility should be reviewed as part of the same planned maintenance.
Use a concrete readiness list
For example, an administrator with users named Alex and alex should resolve that conflict deliberately and preserve the relevant account data before proceeding. Record the current server version, backup and intended scan, then follow the release’s exact supported path. Keep the source media and manual library information intact while migration runs; do not interrupt a working data migration merely because it takes longer than an ordinary restart. After the required scan, inspect known versions and user access. These checks are specific to the documented 12.0 transition and should be revisited against the installed release instead of copied indefinitely into later upgrade instructions.
Questions, answered
Can two usernames differing only by capitalisation affect this upgrade?
Yes. Jellyfin’s 12.0 notes identify that collision as a migration failure condition.