The downgrade option should reduce the pdfile version on all databases which have been upgraded and remove all updated structures, (any text/2dsphere indexes) so that the databases can be started/used with 2.2.x without error. This option will not need to do a repair for a downgrade from 2.4 for example.

The current workaround is to run repair on the downgraded binaries (so 2.2.x for a downgrade from 2.4.x) which implies that the previous version can read and do a repair on the newer file format (higher pdfile version) which is a confusing and unexpected behavior.