If you are upgrading the kernel manually or implementing an entirely different kernel for virtualization, chances are there that the server won't boot it.
If the server has KVM, you can select the old kernel on reboot. If KVM is not available, you would need to wait for the Data center to take actions.
The best way to do that is to set the grub command line to load the new kernel just once. And also set the grub.conf default kernel to old one.
[root@myloth ~]# grub
Probing devices to guess BIOS drives. This may take a long time.
GNU GRUB version 0.97 (640K lower / 3072K upper memory)
[ Minimal BASH-like line editing is supported. For the first word, TAB
lists possible command completions. Anywhere else TAB lists the possible
completions of a device/filename.]
grub> savedefault --default=0 --once
savedefault --default=0 --once
Where 0 is the new kernel in the kernel list for me.
If the server won't boot, reboot it and you will be back to old kernel.