Teff, back to the original question. i just got a 13' X5M and have a 21' 128 wheel set from the previous x5 i had. i wanted to put the 21' rims on the new M. The rear rim fits no problem, even though i had to use a centric ring, but the front rim will not even set onto the hub because the M caliper is too big. Its not a height problem, but the caliper is so fat that it bumps into rim. So if you upgrade the brakes, you'll probably have to change wheels as well to accommodate the caliper, not just diameter size, but design as well.