3.3.1. Migration from ext3
Moving to ext4 must be done with a freshly formatted ext4 file system. Migrating in place from ext3 to ext4 is not supported and will not produce many of the benefits ext4 offers, since the data currently residing on the partition will not make use of the extents features and other changes.
Red Hat recommends that customers who cannot migrate to a cleanly formatted ext4 file system remain on their existing ext3 file system.