I guess you are the kind of person that thinks mailing yourself somewhere in a big box and then opening the box at the destination, with the crowbar you carefully packed inside, is a good idea ....... only to find out that you forget air holes ... or someone taped the whole box to ensure it doesn't come undone .... or that there is no-one at the other end to sign for it ... so they pack you in a warehouse with 1000 other fools that did the same thing, er I mean, parcels, thereby ensuring that your exit is totally blocked.
That's the wonderful thing about the real world, you can do all sorts of stupid things & no-ones gonna stop you ....
First of all .. every file that setup needs to replace .. is in use .. and yes MS says they will be replaced when you restart ... but are they????
I've done an upgrade 3 times from Home->Pro from windows (just cos I could) and all three ended up with an unusable OS.
The second best way to upgrade an OS is to boot from the Upgrade OS disc & repair / install/upgrade from the setup menu. Thereby ensuring that nothing is in use.
The best way to upgrade is to delete the old one, and just use you old CD to prove you have it & install a completely new OS