I personally go with the "changing the nature" of someone route. Magical Plastic surgery would only be a Law Breaker if they changed the Nature of that person. Willingness only matters if you are employing a talisman or some similiar object, since using such things involve exercising one's free will.
Think about like this, you look yourself in the mirror everyday and you recognize that image. A lot of people exercise, use makeup, costumes and what not to alter their image but when you look in the mirror, you still recognize the reflection as you. Giving yourself new hair, nails, skin tone, teeth, etc wouldn't do it. Changing those things has become acceptable to modern folks. People's self-image changes when they get braces. It would break the Law when the person changed looked in the mirror and no longer recognized the face in the mirror. That's the point when you've changed someone's nature. If you used magic to simulate the effect of braces in minutes, that wouldn't do it. What would would be when changing that person's smile made him/her no longer recognizable, like an idealized smile, the stuff of every toothpaste commercial when originally you had teeth with dime sized gaps between them.
But the magical dentistry thing is a bit of an extreme case. Warlock by dentistry? not my idea of a Warlock.
Unless the the Black Magic Dentist gave the guy a shark-grin, then that's definitely a Law Breaker.
