You cant. You need the key so you can open the room If you cant get into the room you cant change him you might want to try hacking if you didnt get the mystery gift ;)
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mystery gift or action replay.
In order to change it's form you must receive the "rotom key" through a mystery gift or Action Replay. I got mine at an event at Toys'R Us. Then you must go int a secret room in the Team Galactic Eterna Building. Search for the secret room, and inside it there are appliances. If you have rotom in your party, you just walk up to an appliance, and press a. The rotom will change.
mystery gift is for the amaica rocks and i am still tryiny to figure out the other parts
You can't.
Using an action replay.