Only 64-bit systems are supported in Red Hat Enterprise Linux 7. If you have a 32-bit system, an in-place upgrade is not supported. However, you can use the output from the Preupgrade Assistant to reach a configuration and package set suitable for migrating to a 64-bit Red Hat Enterprise Linux 7 system. All the necessary information for migrating the system to a 64-bit version is available in the /root/preupgrade/kickstart/ directory.
