A previous boot option,
upgrade, has been superceded by a stage in the installation process where the installation program prompts you to upgrade or reinstall earlier versions of Red Hat Enterprise Linux that it detects on your system.
However, the installation program may not correctly detect a previous version of Red Hat Enterprise Linux if the contents of the
/etc/redhat-release file have changed. The boot option
upgradeany relaxes the test that the installation program performs and allows you to upgrade a Red Hat Enterprise Linux installation that the installation program has not correctly identified.