tunefs is used to switch filesystem from ext2 to ext3 and vice versa
tune2fs -j /dev/hda1 will switch ext2 to ext3
tune2fs -O has_journal /dev/hda1 will revert back to ext2 file system
also, when you revert ext2 to ext3, there r bits set in the file system that tells linux when to do a forced fsck on the disks. since ext3 doesnt need this, you can run tunefs -i 0 -c 0 /dev/hda1 to turn off forced checks.