I was one year into the PhD program before quitting and going into dental school. Most schools didn't really care that I was quitting. I went to a couple of interviews, and none of them asked about it. Just do it secretly and don't let your PI know.
It's your life. You do what you want with it. No one can tell you that you have to do this or that.
Specializing is about how well you do relative to your dental school class. Having a masters helps, but grades and rank count for a lot more.