When you say you are replacing the rear bumper, just know that if you are thinking of getting an M sport rear bumper there's a lot more involved. If getting a standard 50i rear bumper, it should be a pretty straight forward swap.
Btw, another option is upgrading the standard tips to 4" as seen in this thread:
http://www.xbimmers.com/forums/showt...ht=35d+exhaust