PhotoStructure versions are always backward-compatible, but not forward-compatible.
In other words, your library is automatically upgraded when you run newer versions of PhotoStructure, but if you then try to run an older versions of PhotoStructure, it won’t know how to “downgrade” your library to the older version.
If the upgrade caused a rebuild, your database assets won’t point to the correct previews (as those primary keys have changed), so you’ll need to delete your library’s .photostructure directory and re-import everything.
If your library wasn’t rebuilt, you’re in luck: PhotoStructure makes a backup of your library database when it does an upgrade, and makes backups periodically as well.
First, shut down PhotoStructure
Second, follow these instructions to find where your database backups are and restore an older backup. Copy (don’t move) a database backup from before you upgraded to $your_library_directory/.photostructure/models/db.sqlite3
Finally, downgrade PhotoStructure.
If you’re using PhotoStructure for Docker, change to photostructure/server:stable (rather than :alpha or :beta) and docker pull or docker-compose pull the image.
If you’re using PhotoStructure for Node, run git checkout main ; ./start.sh
If you’re using PhotoStructure for Desktops, just re-download the installer.