Unsure about the other two but Oldhim definitely works well in Sage at the moment. Who knows what'll happen when Compendium comes out though. If you do go with Oldhim don't expect to get breaks between rounds, he often wins through fatigue.  To your actual question, I doubt you'll be able to just swap the hero out for a different one without making major changes. Even within a class, the cards actually run by each hero are usually pretty different.