The OS migration can be done in place: (redhat linux 4.5 to 6.x)?
Upgrading from a previous Red Hat Enterprise Linux release to RHEL 6
Red Hat does not support in-place upgrades from one major RHEL release (e.g. RHEL 5) to RHEL6 (see https://access.redhat.com/knowledge/solutions/21964). The only supported option to upgrade from an older RHEL release to RHEL 6 is to backup all data on the system, install and configure RHEL 6 on the server and then reinstall the applications and restore the data from backup.
1496410 - Red Hat Enterprise Linux 6.x: Installation and Upgrade
Regards
RB