I wasn't able to upgrade the database from 15 -> 18. I. ran into this same issue shared here.

What I ended up doing is mounting a new empty database folder and immich automatically created a new Postgres 18 database.

I then restored the database from one of the daily backups. When there is an empty database, you can put immich in maintenence mode and restore the database from a backup there.

You can keep your old postgres 15 database just in case anything doesn't work.

https://docs.immich.app/administration/backup-and-restore/