Helps if we understand why you need to change the license type. Is this so you can offer the model as royalty free for personal use and additional fee for commercial use?
If the above example is true, there is not really an option for 'variations' on the CGT platform so you would have to make a duplicate and offer it as a different price, however it would be obvious people would just download the cheaper version as the tags etc would put the model together in search results (most likely).
I'm just speculating so best to know more about your situation.