Where the wind meets the bumper is different on the 2 to 2b. The mark 2 is a straight line, the 2b has a sloping line. This may cause you some problems. As also said, you would need to do some modification to change the shap for the lights.
LOL sorry iv just read what i wrote lol i mean the rear bumper, Anyways I won one on EBAY for £2.10 and it looks like it's going to be a stright swap. i just need to by sum of them 5 point drivers.