In a recent sealed queue I entered, my opponent had a Soul Seizer on the board, and a Cackling Counterpart in his hand. Soul Seizer has the following ability:
When Soul Seizer deals combat damage to a player, you may transform it. If you do, attach it to target creature that player controls.
What would happen if my opponent targeted the Soul Seizer with his Cackling Counterpart, and then damaged me with the token?
From this official article on double-faced card rules:
If an effect copies a double-faced card, it copies the characteristics of whichever face is currently showing. It doesn't copy any characteristics of the other face.
The only objects that can transform are cards that physically have two faces. If a token or a card with a regular Magic back is instructed to transform, instead nothing will happen. Even if a token or non-double-faced card is a copy of one face of a double-faced card, it can't transform.
If your opponent used his Cackling Counterpart to copy a Soul Seizer, he'd better hope he had a good use for a plain old 1/3 Flyer, because that's all he was going to get!
