If you ever need to change the user name or the serial number of your Mac Office…

remove these two files:

Office 2001:

HD/System Folder/Preferences/Microsoft/Microsoft Office Settings (9)
HD/Applications (Mac OS 9)/Microsoft Office 2001/Office/OfficePID

Office v.X:

~/Library/Preferences/Microsoft/Microsoft Office Settings (10)
HD/Applications/Microsoft Office X/Office/OfficePID

Office 2004 (SP1):

~/Library/Preferences/Microsoft/Microsoft Office Settings (11)
HD/Applications/Microsoft Office 2004/Office/OfficePID

(~ is your home directory)

The OfficePID file is hidden, so the best thing is to delete it via terminal.
The next time you start a Mac Office application, you’ll be prompted to re-enter user info and serial number, just like after the installation.

Oh and if anyone knows how to retrieve the used serial number, let me know.