Move Deleter and Relearner
The move deleter can delete any move from your Pokémon. That is, even HMs. The move deleter in the house right below the Pokémon Center in Canalave City. This is useful for deleting HMs off of a Pokémon that you want a better move on, such as Defog or Cut. HMs page.
The move relearner is in Pastoria City. He can get your Pokémon to relearn any move it has forgotten. They can even learn new moves this way, such as Shadow Claw on a Charizard. The catch is that you need to bring him a Heart Scale. Luckily you can find some Heart Scales easy Underground.
Good luck on raising good Pokémon with new moves, old moves, or some of both!