During a battle, one of your Pokémon can undergo Terastallization at any time you choose, allowing it to change its typing for offensive and defensive purposes and gain an advantage in battle! In battle, the effects last until your Pokémon is defeated, even if you switch it out the field and send it back in later turns.
No, it's like Megas. Your Terastal mon will still have its Tera Hat if you switch it out and back in again.
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