Just create a new partition and copy the data. Then update /etc/fstab to include a new line; say for example (adjust as necessary)
/dev/hda10 /var/www ext3 defaults 0 2
This will mount everything in directories "below" as well, so be sure to copy everything over; I like to use "cp -a <target> <source>" from a liveCD (recovery mode from the boot menu should be o.k. too).
Do this on your current system to ensure nothing gets "lost".
While you're in the mood, why don't you do likewise for the /home, so you don't lose that in the upgrade.
Just did similar on a Dapper to Feisty jump, although I used a new partition for the root; I think you'll need to use the alternate CD to assign your own mountpoint(s). And you'll probably need to do the fstab change for /var/www by hand again after the new system has been installed.
Whoa, you must type fast ...
All good, I'll go back to sleep.