You should be able to convert to ext3 no problem, just use:

tune2fs -j /dev/hda1 (make sure to use your hd device)

I think you have to start off with rieserfs if you want it, the latest release(s) ask in the hd-install if you want ext2, ext3, rieser or xfs file system(s)

with a 2.4 kernel I think the kernel will start kjournald when you boot-up

rock