If you want to use Imperial as your native units then I would do that as the first step. You could do it two ways, the first is just change the native units to imperial and then recalculate the steps per unit and retune the motors. The second would be to make a new profile and do the same as above. Peronally I think the new profile would be best as it would mean you still have your old profile to fall back on if you hit a problem.
Hood