You should have, because downgrading is definitely not supported and can break your setup, so before trying to downgrade Owncloud, I would rather try to “force” an upgrade from 10.15, which is also not officially supported, but probably much less risky. See also this thread: Upgrade from Owncloud 10.15 to Nextcloud
Whatever you do, make sure you do a full backup first, preferably a snapshot or backup of the entire system, so that you can easily roll back if things go sideways.