If I remember correctly you have to return the tune stored in the hypertech to stock before allen tunes it. Once he tunes it you can reload the new tune into the hypertech, modify the tire size and then download it. I have read where others have done that to sell the unit to others. If you don't return the stored stock tune back to the PCM the hypertech thinks you are trying to tune more than one vehicle with the unit, which it doesn't allow. The hypertech unit doesn't know if you've changed the stock tune or not, it just stores the programmed data. It would be impossible for them to store all the possible iterations of factory tunes in the unit for comparison.