Getting a bit technical, but from what I understand you must have had a Validated Copy of the previous OS installed. If you format, I understand, that there are still markers that MS recognizes to know you DID have a validated copy at one time. The OP, unless I am mistaken, never had a validated copy.
Actually, with Seven, you choose a clean install the format is automatic, but Big Brother still knows.
Bottom line, due to my understanding, I would be very reluctant to tell the OP to buy an upgrade version, since I fully believe that it would not validate.