Upgrade, then downgrade and upgrade again

We are testing the downgrade from version 5 to 4 as a backup plan in case our upgrade goes wrong.
When doing the downgrade the index’ are rebuild as described in in v5 release notes.
But the new $by_correlation_id in v5 projection is marked as faulted.
We tried to delete it and it seemed to succede.
But when we tried to reupgrade the same eventstore we hit the following issue for the $by_correlation_id projection and it does not respond to reset or delete commands.

Multiple projections emitting to the same stream detected. Stream: ‘$bc-2085c389-8cd7-56bf-8f4c-c4d7c8ec5b6f’. Last event projection: ‘73’. Emitting projection: ‘75’