For various reasons you may need or want to downgrade your Windows 7 Ultimate to Windows Professional, but at the same time you don’t want to mess up any of Window’s formatting or potentially lose your data in the process. There are several tutorials on the web that explain how to downgrade Windows 7 Ultimate to Professional, but most of them require a fresh installation in order to work.
Below I’ll be outlining a way for you to go through the downgrade process easily.
1. Before you start the installation process change the registry keys “EditionID” and “ProductName” under the Data column to “Professional” from Ultimate and “Windows 7 Professional” from Windows 7 Ultimate respectively. If you are wanting to downgrade to Windows 7 Home Premium then you’ll want to rename the data for those files to “HOMEPREMIUM” and “Windows 7 HOMEPREMIUM” respectively. (See images below)
Notice the differences under the Data Column.
Note: It is important that you match up identically the version name used with the installation DVD with the renamed data files.
2. Now insert the Windows 7 Professional installation CD and choose the Upgrade installation type.
3. After the installation process has been finished you should now see the welcome screen corresponding to the version of Windows 7 that you downgraded to.
All of your files should remain intact through the process. There are a few minor settings that will be reset with the new version, but those can easily be changed to fit your needs.
For more images on the downgrade process check out the source.