Windows 7 does not allow users to “downgrade” or convert from a more premium and more expensive edition to down-level, more basic and less expensive edition. For example, it’s impossible to downgrade from Windows 7 Ultimate to Windows 7 Professional, or from Windows 7 Professional to Windows 7 Home Premium or Home Basic.
Windows 7 SKUs or editions have a hierarchy as below:
Starter → Home Basic → Home Premium → Professional → Ultimate ( → Enterprise )
Upgrade is only possible one-way from the lower edition to higher edition of Windows 7, and not the other way. If user attempts to perform an in-place upgrade to downgrade to a lesser edition of Windows 7, an error similar to below will be displayed at compatibility report: