Once something is released, it’s a release. You can’t unrelease something. As unwieldy as Classical Archives’ old policy was, it’s a fact that should be reflected. I wouldn’t advise anyone to add all these old releases though. These are releases with a very limited distribution.
So I would keep the old releases as they are in MusicBrainz now, add the new “full” releases if they aren’t there already, and merge the recordings.