I'm in that position right now. I accepted an offer at the only school I was accepted at, but got off the waitlist at a school with a better program and that I'd rather go to so I plan on rescinding my commitment. I talked to some professors at my current school and they said about 1 or 2 people do it every year. There's also 3 first year students I talked to who accepted offers and backed out for a different program. So with that being said, I would say it happens fairly often, but it all depends on the school and the program.