It all depends on the way you have registered the application. If you have downloaded a pre-activated product, you can do the same using the email that was provided after you have made the purchase. Contrary, if you used a serial number, just install the free version then use the details to activate it. These are the proper methods for this issue. In case, the serial key has limitations regarding installation procedures, you might need to contact the seller of the application to issue a new serial key.
