Once during your turn (before your attack), you may search your deck for another Unown and switch it with Unown. (Any cards attached to Unown, damage counters, Special Conditions, and effects on it are now on the new Pokémon.) If you do, put Unown on top of your deck. Shuffle your deck afterward. You can't use more than 1 Shuffle Poké-Power each turn.
If your opponent's Bench isn't full, look at his or her hand. Choose 1 Basic Pokémon you find there and put it onto your opponent's Bench. Then, switch it with the Defending Pokémon. Your opponent chooses the Defending Pokémon to switch.