When upgrading or installing a new Kernel version from a RPM package, the first thing you have to know is what Kernel to use. See this page for more details about discovering the right version.
Always try to keep the current Kernel installed when upgrading, so you can test the new installed image an see if you'll get in trouble for some reason, if this happens so you can reboot with the current one. Starting from this point, we can say: Never use the "Freshen" or "Upgrade" commands unless you really knows what will going to happen. When upgrading critical packages like Kernel, try to use the Test option before executing the final command. And always try to install, not upgrade.
And finally, when upgrading, use the Backup option "--repackage" so you can reinstall the old package you've removed during upgrade process. You can issue the command below to upgrade the Kernel and making a backup of the current and installed Kernel package:
rpm -Uvh --repackage new-kernel.rpm
The Old RPM will be generated on "_repackage_dir" RPM macro, usually /var/spool/repackage, check this for more details on macros. If some problem occurs with the new installed Kernel version, then use the following command to reinstall the old one:
rpm -ivh --oldpackage /var/spool/repackage/old-kernel.rpm