The bolt carriers are different lengths between generations (as are the corresponding barrel extensions). They can't be mixed and matched. The feed ramps are slightly different as well (pretty subtle), but you can install a gen 2 feed ramp in a gen 1 gun if you want. To my eyes the uppers appear to be identical between the two gens, and from experience gen 2 uppers work fine on gen 1 lowers.
Thanks for the info..good to know