Can't be done, but you can be a visiting student your third year, wherein you take classes at a different school but receive your degree from your original school. A lot of people do this just to have access to networking opportunities in that final year if they're absolutely committed to a particular region.
As others have said, academia is hard to swing no matter where you go, but if you really want to shoot for it, your best bet might be to graduate from BC and apply to LLM positions at elite schools. Nearly all of my professors have LLMs in addition to having gone to Yale/Harvard/Chicago/etc.