It happens more often than you might think: you have two Office licenses and install the wrong one on a computer. If you activate it, Microsoft will think you moved it to a new computer and it can create problems later on.
How do you change the installation key without reinstalling Office?
Close all Office applications that are open. Open the registry editor by pressing the Windows key + R and typing regedit in the Run dialog then press Enter.
Find the following registry key (were xx = your version of Office) and locate the correct key for the application that needs a new installation key ("CD key").
The Office suite and any applications you installed individually will each have a registry key, identified by a GUID which resembles this value:
Look at the data values for the application name, then delete the two ProductID keys – (DigitalProductID and ProductID).
Open the application (any of the Office applications if you changed the suite key) and enter the correct CD key. If you already activated Office, you'll need to reactivate it using the correct key.
If you have more than one Office license, after activating the correct copy on the correct computer, save the activation files and export the Registration registry key. When you reinstall Office, you can use any of your licenses to install it then restore the activation and run the exported *.reg file to restore the correct licensing information on that computer.
Learn how to back up your Office activation at Tip 53: Saving your Office Activation.
Published February 14, 2011. Last updated on November 28, 2012.