I don't know if this is the most efficient way, but the way I had to do it when changing my product key (I thought I would just use my roommate's product key... but alas, it didn't work) was uninstalling Office 2008, deleting everything in ~/Library/Preferences/Microsoft, and then reinstalling Office 2008.
I don't know if this is the most efficient way, but the way I had to do it when changing my product key (I thought I would just use my roommate's product key... but alas, it didn't work) was uninstalling Office 2008, deleting everything in ~/Library/Preferences/Microsoft, and then reinstalling Office 2008.