Since some of you may go for a clean install of 10.2, here's a little trick. If you are partitioning your disk (and you have to for this trick to work; it won't work on a single partition), install Mac OS 9 on your first partition and install OS X on any other partition. After, select the partition or disk with OS X as the startup device and leave it there.
If you ever, for some reason, have to boot into OS 9, simply hold the D key at startup right after the chime. If you reboot, you'll have to hold D again otherwise you'll end up in OS X.
For your information, on startup, the D key tells the machine to boot on the first internal bootable partition or disk in the computer, whatever the startup disk is (useful if you're stuck on a CD). In this case, since OS 9 is installed there, it boots OS 9.
The only thing is that this trick may not be that useful in the future when OS 9 won't be bootable (not that I mind...).
Mac OS X Hints
http://hints.macworld.com/article.php?story=20020816093751806