I have the same problem.  I got my copy of 6 Pro second hand on eBay, and now I can't get the upgrade.  I contacted Daz and they were no help at all.  I CANNOT afford the full version price for 7 pro so I may be pooched...  They didn't seem all that enthusiastic about keeping a customer.  Now I am stuck with a buggy install of 6 which I won't even have when I build my next box.  I can't find my receipt or original disks even...