Go2Linux posted a tutorial about upgrading from ext3 to ext4 file system
This howto will show you how to upgrade from ext3 file system to ext4 file sytemHow to upgrade (convert) ext3 to ext4 file system
Please be aware that following this instructions may destroy all your data, so you are really encouraged to back up all your data before continuing.
Also, before doing any conversion, be sure you are really going to notice some noticeable improvements.
