Pokémon Ruby Version, Sapphire Version, and Emerald Version - Learning new moves
Sometimes, after a Pokémon levels up, it could 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 is still changed to 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.
Mimic and Sketch will take account of the target’s last move used even if the target no longer has that move. Also, if the move changes this way, this doesn’t stop any two-turn attack, Hyper Beam, Bide, Thrash, Rollout, Uproar, and equivalent moves in progress. Grudge and Pressure will reduce the PP of the newly used move as though that move were chosen for use.