After the upgrade, I found that the Classic Preferences couldn't see my 9.04 folder; it just saw the 9.1 folder, which meant I couldn't use Classic.
I restarted into 9.1, and then used the System Disk utility to select the OS X partition's OS 9.04 folder as the boot drive, then rebooted into 9.04. Once there, I again used the System Disk app to select OS X, and booted into OS X. I was then able to specify the 9.04 folder as the Classic environment in the Classic prefs dialog.
It seems OS X 'remembers' the last OS 9 folder you used, and uses that one as the Classic environment. This can be a bit dangerous if you have two 9.04's on your machine, and you're trying to keep one 'clean' from OS X. I haven't tested it, but it appears that if you simply boot into your 'real' 9.04 and then launch OS X, you'll change the Classic environment to your real OS 9.04. It would appear the safest way to do this would involve running whatever you needed in your real environment, then booting into the OS X 9.04 system, and then booting into OS X.
Hopefully this will all be moot point when OS X 1.0 ships.

