If Windows 7 works the same way as Vista, you can use the upgrade disc to install the full version of Windows. It's an exploit that many people took advantage of, me included.
Perform at your own peril. It may not work for 7. Here's a link explaining how it was done with Vista.
http://www.winsupersite.com/showcase...rade_clean.asp
I would never do anything but a CLEAN FRESH install for a new OS. I can't trust anything else.