When the RHEL 6 system contains several kernels, after the rollback only the one that have been active when the upgrade process started is restored. The others are installed but system cannot boot into these, because because kernel files in /boot/ are missing. From this reason the entries of such kernels are removed from the original grub as these cannot be used safely. However the RPMs are installed.
To make it possible to use other installed kernels as well:
backup all those files
reinstall the other kernels after the return to the original system
When the RHEL 6 system contains several kernels, after the rollback only the one that have been active when the upgrade process started is restored. The others are installed but system cannot boot into these, because because kernel files in /boot/ are missing. From this reason the entries of such kernels are removed from the original grub as these cannot be used safely. However the RPMs are installed.
To make it possible to use other installed kernels as well: