"Correction: only one thing can change a face and change it back. Fire, surgery, a punch to the nose, those change faces as well."

But only the fae can swap between faces like humans swap between clothes. This is just the Doctor being difficult. Though there's a new layer added to his difficultness as he pauses, thinks of something else, then continues with a shrug.

"Well, us and weres and shifters. An animal face is still a face."