If you attempt it, you’ll receive an error message like this:

Microsoft has declared that you can only upgrade from Pro-to-Pro or Ultimate-to-Ultimate, meaning if you want to go from Vista or Windows 7 Ultimate to Windows 7 Pro or Windows 7 Home Premium, you have to do a fresh install, thereby losing  all your settings, favorites, installed programs, etc.

In my case, I was trying to upgrade my wife’s computer from Vista Ultimate to Windows 7 Professional. But it’s also as big problem for anyone who ran a beta or pre-release versions of Windows 7, which only came in the Ultimate flavor.

But, of course, there’s a hack.

You can trick the Windows 7 installer into thinking that you’ve actually got a lower edition installed, which will then allow the upgrade to proceed. Here’s how:

• Open the Registry Editor (run regedit.exe).
• Go to HKEY_LOCAL_MACHINE\Software\Microsoft\Windows NT\CurrentVersion

• Now you need to change the EditionID and ProductName keys to a lower edition of Windows.
• To upgrade from Vista Ultimate to Windows 7 Professional, change the EditionID from Ultimate to Business, and the ProductName from Windows Vista (TM) Ultimate to Windows 7 Business. You read that right – even though you’re running Windows Vista, you can trick it into thinking you’re running a version of Windows 7 that is eligible for upgrade.
• To downgrade from Windows 7 Ultimate to Windows 7 Professional, change the EditionID from Ultimate to Professional, and the ProductName from Windows 7 Ultimate to Windows 7 Professional.
• To downgrade from Windows 7 Ultimate to Windows 7 Home Premium, change the EditionID from Ultimate to HOMEPREMIUM, and the ProductName from Windows 7 Ultimate to Windows 7 HOMEPREMIUM.
• To downgrade from Windows 7 Ultimate to Windows 7 Home Basic, change the EditionID from Ultimate to HOMEBASIC, and the ProductName from Windows 7 Ultimate to Windows 7 HOMEBASIC.
• Run setup.exe from the Windows 7 DVD (don’t boot from the DVD).
• Choose the Upgrade option when prompted.

 If you’re trying to “downgrade” from the Windows 7 Ultimate Release Candidate (Build 7100), you have to jump through a few additional hoops:

• Copy the contents of the install DVD to a new folder on your hard drive.
• Locate the cversion.ini file in the Sources sub-folder, double-click it to open in Notepad, then change MinClient=7233.0 to MinClient=7077.0
•  Run setup.exe from the folder on your hard drive, and choose the Upgrade option.
• When the upgrade is complete, you can delete the folder.

Keep in mind that in all of the above cases, you’ll need to re-activate your copy of Windows with the new Product Key once the upgrade is complete.

Happy upgrading or downgrading!

Related posts:

1. Activate Windows 7 God Mode
2. Windows 7 In Effect
3. Using Windows Live Family Safety with a Windows Domain