I'm not saying it's wrong, I'm just saying it's very, very difficult. You have to justify it to both schools -- and if your reason is that the new school is better, the one you're leaving will be offended, and the one you're going will question why you weren't accepted there in the first place. When you apply for residencies you will have to answer questions about the transfer -- why you transferred, what you didn't like about the first school, etc. If it's done for a frivolous reason (which is actually hard, because frivolous reasons usually aren't rewarded with successful transfers), you could be seen as someone who's easily dissatisfied, hard to please, thinks the grass is always greener, etc. (On the other hand, you might be able to spin it as a quest to better yourself.)