I have also heard that it is very, very hard. Dental school is not like college where you pick and choose your classes. There is one basic curriculum that everyone in a school goes through together. And different schools teach different things during different years so transferring isn't as easy as picking up where you left off or taking a few classes on the side to catch up. Additionally, if no first years drop out of the school you are hoping to transfer to there will simply not be room for you.
Moral of the story: plan on being at one school for the whole four years.