Parallels 6: Upgrade the upgrade?

I had an older Parallels license on hand, which I thought was from version 4. So I bought the upgrade version of Desktop 6, and when it came time to activate it I entered my older key and found it invalid.

So now that I know that my older key is not from 4 or 5, is there any way I can pay the difference and upgrade my Parallels Desktop 6 upgrade key to a full key? Or does Parallels expect me to eat the $60 and buy a whole new key?