[Natanael_L]

You can retroactively do anything that doesn't change the status of existing copies held by others under valid licenses that don't allow you to force a licensing change.

You can remove GPL from something you yourself put GPL on, but existing copies held by others are still under GPL.

You can make something Apache licensed that was proprietary too, and old copies will remain proprietary unless you declare an exception valid for all existing copies under your copyright (which may be what you meant?).