>Rotom's five alternate forms make a return within this game. When you have sent Rotom over, you will be able to take Rotom with you to the Department Store in Route 9. In this store, there is a room with multiple storage boxes. Interact with this box and you'll be able to change your Rotom's form. These Rotom all have special attacks that can only be used while in that form and they change type.
Go to the Department Store on Route 9, find the room with storage boxes and interact with the boxes.