Firtst of all make sure ext3 is compiled in the kernel otherwise it'll never work
Next make the partition ext3 compatible (for example I'm using /dev/hda1) using tune2fs:
tune2fs -J /dev/hda1
Now edit your /etc/fstab and change the ext2 word to ext3. For example:
/dev/hda1 / ext3 noatime,defaults 0 1
Now remount your partition or reboot.
You can use tune2fs while your partition is mounted so you can safely do it. If you don't want to use ext3 anymore, just change the /etc/fstab and you're done...
Hope this helpes.
