Note: The following steps involve modification of registry key values, which may corrupt your Windows if done incorrectly. Backup registry first before performing the workaround.

Close all Microsoft Office programs.
Click on Start button, then click on Run.
Type “regedit” (without quotes) in the Run text box, and click OK or press Enter.
Locate and then click the following subkey:
HKEY_LOCAL_MACHINE SoftwareMicrosoftOffice12.0Registration

Inside, you will find another subkey that resembles the following subkey:

HKEY_LOCAL_MACHINESOFTWAREMicrosoftOffice12.0Registration{30120000-0011-0000-0000-0000000FF1CE}
Optional: Backup this registry branch by exporting the Registration subkey to a file, just in case the new product key does not work and you have to restore back the old product key. To export the registry, right click on the Registration subkey and click on Export, and follow the on-screen prompt to enter a file name for the registry file and choose a location to store it.
Under the Registration subkey, there may be several Globally Unique Identifiers (GUID) subkey that contain a combination of alphanumeric characters. Each GUID is specific to a program that is installed on your computer.
If you find additional subkeys that reference Microsoft 12.0 registration, then click and open each GUID subkey to view and identify the Office product version by the ProductName registry entry in the right pane. For example:

ProductName = Microsoft Office Proffesional Plus 2007
After you find the GUID subkey that contains your Office product or program which you want to remove the existing product license key or registration details, delete the following registry entries by right clicking on the registry entry in the GUID subkey, click Delete, and then click Yes:
• DigitalProductID
• ProductID
Exit Registry Editor.
Run or open an Office application program, such as Word or Excel or Outlook. Office 2007 will prompt you to enter a new 25-character product key.

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How to change Licence Key for MS Office 2007