Pokémon Diamond Version, Pearl Version, and Platinum Version - Learning new moves
Sometimes, after a Pokémon levels up, it may learn new moves during a battle. If this happens, the new move is copied and replaces the old move that used to be there, unless Transform is in effect for the Pokémon or the current move is different from the Pokémon’s original move (in which cases the Pokémon’s original move still becomes the newly learned move). When the Pokémon’s attack segment comes (if it hasn’t taken its attack segment already), the Pokémon will use the move located at the old move’s position.