Is it possible to downgrade editions without clean install?

I am currently running Win 7 Ult 64 on my desktop.

I am building a new system & want to use my Win 7 Ult key on it. From what I've read, there is no reason I can't move it to another machine (it's not an OEM copy). I want SUA on my main PC.

The reason I'd like to downgrade is to use my old box for HTPC tinkering and whatnot, and I already bought a new edition of Win 7 to use for that purpose (still will use 64 bit). I *could* do a clean install, but it would be nice to have my settings saved, etc (nothing getting recycled from old box except one data HDD). I don't *need* Ultimate to tinker with a HTPC, and not being made of $$, I don't want to buy another Ultimate copy again.

There is a workaround to change between versions which was used by many to change from beta to their paid lesser versions, however I don't know how SUA would fit in. What is the version you wish to end up with exactly?

The workaround requires changing 2 Registry listings to trick the installer into thinking it's doing a same-version Repair Install which in Win7 is an in-place Upgrade.

You change the following keys to exactly the lesser version name, e.g. HOMEPREMIUM for the EditionID, and Windows 7 HOMEPREMIUM for the ProductName. Then run the installer from desktop, activate with new key:

click to enlarge

Thanks, I'll try the registry trick first. The SUA part required additional downloads from on top of Ultimate; I don't really remember all that I did to set it up! If it causes problems, I'll just migrate or clean install, depending on how ambitious I feel.