You can "upgrade" from Ultimate to Professional, though it needs some registry hacking.This isn't exactly the point.
When I downloaded the RC we were given Ultimate. But when I got the (free) final version from MSDNAA it was Professional. So I can't use the upgrade option, I need to do a clean install.
I agree, I'll happily use Pro, I just wanted Ultimate so I didn't have to do a clean install.
Tried it myself, "upgraded" from X64 Ultimate to x64 Professional.
Use this guide: http://www.unawave.de/installation/downgrade-en.html?lang=EN
And just so you people know, most of Win 7 (excl. E, N etc) editions are on the same dvd; if a certain edition is automatically installed, it's because of ei.cfg file. Copy the contents of the dvd onto your hdd, delete the ei.cfg file and burn the files onto a dvd. Now you can choose, which edition to install. Also, this file can be edited to install a certain edition, ie Professional or HomePremium.
The ei.cfg file is in the "sources" folder if I remeber correctly. Here's the guide: http://windows7center.com/news/how-to-install-any-version-or-sku-of-windows-7/
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