Without save file editing, you can't reform a second time time or alter the religion once it is reformed.
If Holy Fury was disabled, the reformed religion has no nature, doctrine, or leadership. Its default features will be those of pre-patch 3.0. Eg. Germanics have a temporal religious head, all others have a theocratic religious head.
They are compatible by necessity, since players with Old Gods can play pagan but can't select a reformed religion's nature, doctrines, or leadership.
Hell, the game won't crash from loading a save dated from the release of Conclave. It'll mess up provinces and holdings though. I know because I actually tried it.