From my understanding of the schools I've briefly asked about this, they really don't like doing it, and it's only for really extreme circumstances, or as Beestie said, being required to move around, and even then, it's still hard to do.
And are you sure you want to go to Columbia after having already finished your first year? They've got a very heavy Science class load, more so than most dental schools, and if your dental school didn't match it somewhat closely, they'd probably just tell you that you wouldn't be able to take the classes you needed to get a degree from them. Don't get me wrong, it's an awesome school though.