I didn't fully understand the change that you are explaining, but in general having old & new schemas working relies on the migration definition having the proper `up` & `down` functions defined. These are postgres functions so you can fit any logic in them to ensure that a column deleted in the new schema gets properly backfilled to the old one.

I wonder if there will be cases where this gets too complex or otherwise hits any limits. It may be possible that for those cases a raw SQL migration is required.