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
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!