That's imposing your own set of rules onto something, again :/
There's tapping into something, and then there's changing/turning into something. Plus, he's changing slightly anyway, his hair changes color, his eyes are red and white, an aura coats his jacket blood red, he's got the red veins.
It's not to say that he couldn't grow claws, horns, fangs, wings, a tail or whatever. We seriously don't know more than his current DT design. You have to be patient.
Remember - baby steps. A Caterpillar doesn't just magically change into a butterfly. If you could see all the gross stages it goes through inside its cocoon, you'd understand.