Try doing a search on this subject in the pre-allo and allopathic forums -- it's been discussed extensively. Just for a quick rundown -- transferring during med school is EXTREMELY difficult, and fairly rare. First off, VERY FEW schools even accept transfers in the first place. Secondly, of the schools that do accept transfers, most don't accept transfers until the third year. Finally, to transfer, you usually need a really, really good reason -- fiance or spouse at that school (either as student, resident, or faculty), some compelling family reason (need to be close to home for sick family member, etc.). "I don't like my current school" usually isn't a good enough reason. Also, because only a small amount of spots are available each year for transfers, you of course need to have very good grades and get a strong letter of support from your Dean of Student Affairs.
Don't choose a school based on the hope that you will be able transfer halfway through. If you don't really want to attend the school where you've been accepted, just reapply -- your chances would be better that way.
