Devil wrote:If you have one version fully installed, say XP Home, and you want to change it to a full version of XP Pro, you cannot. I understand it's the same between Vista versions, but I cannot be categorical as I haven't tried it. Once you have a product key encrypted into the registry, it cannot be changed, short of a clean install, of course. I dare say there are workarounds at a registry level, but I don't know of them. I found this out when I accidentally put in a wrong key (had two identical licences but put the same key in both). I realised what I had done before activation and I tried to correct it by a repair reinstall as I had other drivers and software installed by then. It would not accept the second key, no matter what I did. I had to do a clean install. Try it yourself, if you like.
For WinXP, if you want to change WinXP Home to WinXP Pro, you need to do a upgrade install or do a clean install. But for Vista, it's another story. Microsoft has changed the upgrade mechanism for Vista.
Vista uses the same installation disk for all its editions, that's why you will see all Vista editions listed at the first step when you install Vista. The Vista edition will be decided by the edition you choose when install and the Vista product key you have. Different product keys will unlock different features of Vista. You can purchase a small Vista upgrade software which includes a product key online from Microsoft to upgrade your Vista edition without a clean install.
For more information, check the following links.
Windows Anytime Upgrade - Program Detail:
http://www.microsoft.com/presspass/news ... WAUFS.mspx
Wikipedia - Windows Anytime Upgrade:
http://en.wikipedia.org/wiki/Windows_Anytime_Upgrade
Windows Vista - Windows Anytime Upgrade:
http://www.microsoft.com/windows/produc ... rview.mspx
Windows Vista Help: Upgrade to another edition of Windows Vista:
http://windowshelp.microsoft.com/Window ... 81033.mspx
Windows Anytime Upgrade - FAQ:
http://windowshelp.microsoft.com/Window ... 81033.mspx
Hope this helps.
H.T.