There is no such thing as an HD upgrade. The HD license officially is only sold with HD hardware.
In your situation, the only way to get the HD10 license is to buy it from someone here on the forums, or perhaps off Ebay.
Here is someone selling one in the Buy and Sell section, at a great price:
http://duc.avid.com/showthread.php?t=327040
He's a very legit guy who has sold lots of stuff to people here on the forums. You would be much better off buying from him than taking a chance on Ebay.