Here at work we are part of a large Gannet Enterprise agreement with Microsoft so we have all the media we need. There would be no reason to need to change the CD-KEY if you are under an enterprise agreement. Basically you use the same meida disk over and over with the same enterprise agreement cd-key and never get asked to activate the software. As you already know the cd-key is built into the cd so on setup you can't simply input another cd-key as when you go to activate the software it will give you an error.
Only reason I can see you needing to change the cd-key on a media disk is for "other" reasons