Ext3 filesystem access after downgrade from v4.2 to v3.6

[dean.plant at roke.co.uk (Plant, Dean)]

I need to downgrade a system from Centos x4.2 to v3.6 (x86) due to
performance problems with Arkeia Network Backup and AIT-4 tape drives.
The backup database is stored on a v4.2 created ext3 partition. When
accessing this partition after the downgrade, Centos complains on boot
that

fsck.ext3: Filesystem has unsupported feature(s) (/dev/sda5)
*fsck: Get a newer version of e2fsck! [fail]

If I comment the partition out of /etc/fstab the machine boots as normal
and I can manually mount the filesystem.

My questions are:

If I use the v4.2 ext3 filesystem on v3.6 I am likely to corrupt the
data? And what is the best way fix this problem? Do as it says and try
and upgrade e2fsck or is there a better fix?

Thanks

Dean.