yes, it's fairly straightforward, maybe a little scary tho...
to convert ext2 to 3 run
tune2fs -j /dev/hdXX
where XX is the partition number. Then you can edit the /etc/fstab file and change ext2 to ext3 for the relevant partitions. Note that you can only use this on UNMOUNTED partitions, so if you only use one main partition you will need to boot into a rescue mode or such like so as to not have your root system mounted. If you have a /usr or /home partitions you can just go down to single user mode (run 'init 1' on a console) and unmount them with 'umount /usr' for example.
You can still use ext3 as ext2, so if you don't up your fstab immeditaley it won't matter, it simply won't use the journal.