I just looked at the manpage for tune2fs, and I think the proper way to convert EXT3 back to EXT2 is like so. Type tune2fs -O ^has_journal /dev/hda2. Substitute the right partition for /dev/hda2. I just tried this and there was a long burst of disk activity on the partition, so I assume it deletes the journal when clearing this feature. So, this seems to be the way to do it. I'd also edit /etc/fstab to show the filesystem type as ext2, and you may want to unmount the filesystem if possible before running tune2fs as a safety precaution. You can find out a lot about a filesystem with dumpe2fs /dev/hda2. Again, put in the appropriate partition. You should also pipe the output through a pager like less, as it's quite long. You can find out pretty much all the settings of a filesystem from this command.
