Starting with Red Hat Enterprise Linux 5.4 and later, YUM comes with an option to downgrade any packages installed.
In Yum 3.2.22, which come with Red Hat Enterprise Linux 5.4, there is a downgrade option. This will downgrade a package to the previously highest version or you can specify the whole version and release number for the package to downgrade.
Please refer to this page for RHEL 5.4 and above.
Problems with performing downgrades :
Downgrades are tricky but in yum versions 3.2.27 and above it can do _some_ downgrades. They are not perfect and should be used with care
Please refer to this page for RHEL 6.