Can windows vista home premium be installed over the top of vista ultimate?

I have lost my vista ultimate cover with the keycode. I have been getting a screen come up telling me that unauthorised changes have been made to windows and I have to reactivate vista ultimate. I still have my previous version (vista home premium) but it won't accept that keycode.

If you can use the computer try using this program to tell you the activation code of your Vista Ultimate then just reactivate it.

http://download.cnet.com/Keybits-Product-Key-Viewer/3000-2094_4-75547684.html