If you need to change the serial number for Microsoft Office 2008 for Mac here is what you need to do.
- Quit any running Office 2008 programs.
- Delete the following files:
~/Library/Preferences/Microsoft/Office 2008/Microsoft Office 2008 Settings.plist
/Applications/Microsoft Office 2008/Office/OfficePID.plist
Now, when you launch the software, it will then ask for your serial number and you can register it to your own name.