I'm with shah on this one, don't do apt-get upgrade or dist-upgrade, especially if you have done other than 'debian style' HD install. I have had a 'knoppix-style' install survive an apt-get upgrade but only once.

If you do try it; NEVER NEVER NEVER chose 'upgrade to package maintainer's version' when asked if you want to change a configuration script (always keep your existing); expect at least a couple of packages to fail to upgrade & need special attention and expect that the upgrade will set up to start every known service in all runlevels by default. So look at what services you have now in runlevel 5 & be prepared to manually use sys-v-init to restore that configuration after upgrade. The service that causes the most problem is nis so apt-get remove nis before you upgrade (unless you happen to actually need nis).

Also, localepurge & apt-get remove any packages you don't want before you upgrade.