This isn't information that Strange keeps hidden. He's always talked openly about his magic and about what he can do. It's just that he occasionally doesn't remember who he's told what. So sorry Tyki, time to ramble about magic.

"Think of it like running a marathon. I can do it, it just takes time and effort compared to my other magic. I've turned a woman into a cat and a vampire into a bird—though I haven't tried rabbits or dogs yet. Perhaps that will be my next attempt."