Downgrade from 7 Ultimate to 7 Professional

Hi, Guys! I hope you can help me.

A few months ago, I needed to rebuild my system due to the death of a motherboard (the thing had actually warped), so I decided to have my computer guy upgrade me to Windows 7, as I was running Vista Home Premium at the time. We found a dealer selling inexpensive keys, so I decided to go ahead and get 7 Ultimate (x64 version).

Turns out the key I bought was a MSDN version, and was recently flagged, so I needed to buy a new one (idiot me forgot the old adage "if it's too good to be true, it is").

Anyway, due to financial conditions, the best I can afford is a copy of Windows 7 Professional (which has been checked and authenticated). Since I really don't use any of the Ultimate-only stuff, I won't miss it.

I've read several threads here about downgrading using a registry trick to make the computer think it's a repair rather than a fresh install, but I'd like to confirm that it still works. I also read that SP1 needs to be uninstalled before this can be done. Is that still the case, as well?

I have everything set up just the way I like it, and would rather not have to spend three days reinstalling everything...

Well, I've already bought the 7 Pro disk, and I only have the one computer (though I do have a netbook with 7 Starter), so it's all good.

Thanks for the quick reply! I'm glad I won't have to uninstall SP1...