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Downgrading Windows 7 Ultimate to Windows 7 Professional

September 2, 2010

Why do this? When I reinstalled Windows on two of our laptops, I mistakenly did the installation using a Windows Ultimate DVD (and product key). Because this product key was actually from another installation of Windows on one of our home PCs, activation of WIndows 7 failed. I tried to enter a Windows 7 Professional product key, but Windows 7 would not let me downgrade from the Ultimate version to the Professional version.

So a hack was necessary:

  • On existing Windows installation, run Registry Editor (RegEdit).
  • Navigate to the following registry key:

    HKEY_LOCAL_MACHINESoftwareMicrosoftWindows NTCurrentVersion

  • In the right pane, change the value data of EditionID and ProductName to reflect a minor version of Windows 7.

    For example, to fool OS to think that it’s another edition of Windows 7, hack the registry key to the following values as below:

    Windows 7 Home Premium (allow Windows 7 Ultimate or Professional to Home Premium downgrade)
    EditionID: HomePremium
    ProductName: Windows 7 HomePremium

    Windows 7 Professional (allow Windows 7 Ultimate to Professional downgrade)
    EditionID: Professional
    ProductName: Windows 7 Professional

    Note: The ProductName and EditionID should have the same name with the Windows 7 operating system name displayed during OS selection on the installation DVD.

  • Start Windows 7 installation by inserting Windows 7 DVD disc into disc tray.
  • Select Upgrade as type of installation. Then, continue to install Windows 7 as per normal.

Interestingly, the upgrade process seems to have involved copying all the files from the installation DVD and then bringing over settings, installing updates, etc. For this PC, that process took over an hour. Then Windows updates still needed to be downloaded and reinstalled and .Net 4 needed to be repaired (which occurred automatically).


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