Sorry - didn't explain very well. The problem was on booting after the upgrade, not during the upgrade itself. It would get through all the normal bootup stuff, then get to the graphical splash screen, and stop.
After searching online, I have managed to fix it by setting devfsd=nomount in lilo.conf. However, we are now having a different problem, which I think may be related. We have an external 56k modem (dialup - ick!), and when we try to connect via kppp, it says,
"KPPP cannot find:
/dev/modem
Please make sure you have set up your modem device properly and/or adjust the location of the modem device on the modem tab of the setup dialogue."
I'm assuming this is something to do with the change from devfsd to udev, but have no idea what I'm supposed to do about it!
TIA