Downgrading Nextcloud 18.0.3.0 to 17.0.5

I upgraded Nextcloud to the latest save without realizing the gallery app has been replaced by an half-baked “photos” app, completely useless to share pictures in any relevant way to me, in addition to a bug with ublock origin making the whole “sharing” interface disappearing.

Rolling back is not as easy at it seems: the “occ” php app check with your config version if it matches the software version, and if not matching just plainly refuses to work with:

An unhandled exception has been thrown:
OC\HintException: [0]: Downgrading is not supported and is likely to cause unpredictable issues (from 18.0.3.0 to 17.0.5.0) ()

So you need to update this config/config.php. Then, playing with occ app:list, occ app:remove and occ app:install only you can get back to a working install.