Most everything can be swapped. Bodywaork is the same excpet for color and 1200 tanks have the stuff for a fuel gauge, but the tanks will still swap.
The front brakes & forks are different, but whole the front end from a B12 will bolt on a B6. Rear swingarms and their associated parts are different, but the B12 swinger will bolt onto a B6. A B12 engine will bolt into a B6, as will the 600, 750 & 1100 from SACS powered GSXRs and Katanas.