I strongly recommend removing the rotors if you are going to replace the rasp bars. Various reasons why, The tailshafts are probably worn at the bushing area, the bushongs should be replaced, you can't properly shim the rasp bars on the rotor if you replace them in the combine, the rotors should be balanced, and as mentioned - hardsurfacing probably needs to be renewed on the front augers and on the rear separator bars. The small infeed lugs should fit the contour of the auger fairly uniformly. If the rasp bars are not shimmed properly, you will not be able to achieve proper concave adjustments. This can be crucial in crops like spring wheat and flax. Just so you know, the rotor timing can be checked at the rear of the rotors via the openings in the rotor.