I've learned the hard way to always backup the existing itunes files, then perform the upgrade, then restart itunes and let it run all night if needed to update whatever it likes to update. Force quit is not a good idea - better to let it finish what it wants to do.
But I'm getting sloppy. I used to rely on
http://mac.oldapps.com/itunes.php
having an old version of itunes in case I needed to fall back. But that seems to have gone away. I hope this 9.1.1 is a good one. . . In case not, anybody know where I can find a 9.0.3 install image?
Edit: I'm not sure what it is updating - I think it might just be scanning your whole library, and all its music files, to make sure they agree with each other. Letting it do this might prevent problems later . . . Might have something to do with a slightly different library file format (I wonder if they will ever use a real DB program for this)