How can I downgrade from Windows 7 Ultimate to Home Premium without reinstalling Windows? I want to use a Home Premium key and uninstall the Ultimate features. How can I do this?
You cannot downgrade editions. You need to re-install the operating system using the correct edition, then you can use that key.
You can go up in editions using anytime upgrade.
Bonus Chatter: The reasons are dependencies and money. Allowing downgrades means developing a UI, and then endless testing and stomping bugs(this is IMP the biggest pain point). I'd rather the company spend time on features I will use more than once.