You should find info on the RMV website, they have an entire section on "moving to MA" and they definately know how to get their partial taxes. Trust me on that one, I'm a recovered Masshole. 12 years.

The only way I've gotten screwed is moving TO NH, because they don't refund sales tax that gets taken away, or moving to Maine because they don't follow the "6 months = not bought with the intention to operate here" thing that the rest of the modern world uses.

BMW should also be able to help you out if you speak to someone intelligent, but also if you send them a copy of the original sales tax recipt, and you take that in with you when you register your car / get your tags.

IIRC = If I Recall Correctly