Windows 7: Upgrade to Ultimate from Home Premium - Anytime Upgrade or reinstall?

I'm trying to find out the easiest way to go about upgrading from Windows 7 Home Premium (64 bit) to Windows 7 Ultimate (64 bit) and what issues I might run into.

I don't want to reinstall everything (Or anything if possible) so am looking for the easiest option to upgrade.

Anytime upgrade offers the upgrade at $139, a bit expensive.

I found a seller on ebay selling "Microsoft Windows 7 Ultimate 64-Bit Full Digital Download Key " for a lot less (He has about 1500 feedback at 100%)
My question is, can I use the key from this and enter it into the 'Enter an upgrade key' box during the Anytime Upgrade? Will I run into problems doing this?

Alternatively, I could get a copy of Ultimate from Digital River, burn it to a disk or thumb drive, use the key from the ebay seller above and try to do an 'upgrade' install over the top of my current Windows 7 Home Premium install (I don't know if this is even possible?)

Any suggestions are welcome.

Thanks!

The best way for any install is ALWAYS a clean install. Believe me you will have less problems and headaches than an upgrade. It would be worth your while to reinstall your software.

Any anytime Upgrade MUST be done from the existing Windows installation, since almost all it does is make permissions changes within the registry, and add new licenses to the existing install.
The is no way around that.

The 'Digital Download Key' offer is COUNTERFEIT - no-one is allowed to sell Windows licenses in this way except MS itself.

If I have Windows 7 Home (FPP) and Ultimate (WAU) licenses, can I perform clean install of Windows 7 Ultimate and use WAU key ? Will this work or I really have to always install Home and then upgrade ?

Thanks