Answer

One of the safest way to boot RedHat OS with a downgraded kernel is by modifying the /boot/grub/grub.conf

--Get the current Kernel information

[root@host001 nz]# uname -a
Linux car-lnasdwp-001 2.6.32-504.16.2.el6.x86_64 #1 SMP Tue Mar 10 17:01:00 EDT 2015 x86_64 x86_64 x86_64 GNU/Linux

--Edit the /boot/grub/grub.conf

[root@car-lnasdwp-002]# vi /boot/grub//grub.conf

grub.conf generated by anaconda
#
# Note that you do not have to rerun grub after making changes to this file
# NOTICE: You have a /boot partition. This means that
# all kernel and initrd paths are relative to /boot/, eg.
# root (hd0,1)
# kernel /vmlinuz-version ro root=/dev/sda5
# initrd /initrd-[generic-]version.img
#boot=/dev/sda
default=0 ---> change this value to "1"
timeout=5
splashimage=(hd0,1)/grub/splash.xpm.gz
hiddenmenu
title Red Hat Enterprise Linux Server (2.6.32-504.16.2.el6.x86_64)
root (hd0,1)
kernel /vmlinuz-2.6.32-504.16.2.el6.x86_64 ro root=UUID=ffb892af-41e8-4002-beb0-e79d93367445 rd_NO_LUKS rd_NO_LVM LANG=en_US.UTF-8 rd_NO_MD SYSFONT=latarcyrheb-sun16 crashkernel=auto KEYBOARDTYPE=pc KEYTABLE=us rd_NO_DM rhgb quiet
initrd /initramfs-2.6.32-504.16.2.el6.x86_64.img
title Red Hat Enterprise Linux (2.6.32-358.el6.x86_64)
root (hd0,1)
kernel /vmlinuz-2.6.32-358.el6.x86_64 ro root=UUID=ffb892af-41e8-4002-beb0-e79d93367445 rd_NO_LUKS rd_NO_LVM LANG=en_US.UTF-8 rd_NO_MD SYSFONT=latarcyrheb-sun16 crashkernel=auto KEYBOARDTYPE=pc KEYTABLE=us rd_NO_DM rhgb quiet
initrd /initramfs-2.6.32-358.el6.x86_64.img
~
2- Save the file

3- Reboot the host

4- Redhat has booted with the previous kernel release

[root@host-002 ~]# uname -a
Linux car-lnasdwp-002 2.6.32-431.17.1.el6.x86_64 #1 SMP Fri Apr 11 17:27:00 EDT 2014 x86_64 x86_64 x86_64 GNU/Linux

5- You can check the list of available kernels:

[root@host-002 /]# rpm -qa | grep -i kernel
dracut-kernel-004-356.el6_6.2.noarch
kernel-firmware-2.6.32-504.16.2.el6.noarch
kernel-2.6.32-431.17.1.el6.x86_64
abrt-addon-kerneloops-2.0.8-26.el6.x86_64
kernel-devel-2.6.32-504.16.2.el6.x86_64
kernel-devel-2.6.32-431.17.1.el6.x86_64
kernel-headers-2.6.32-504.16.2.el6.x86_64
libreport-plugin-kerneloops-2.0.9-21.el6.x86_64
kernel-devel-2.6.32-358.el6.x86_64
kernel-2.6.32-504.16.2.el6.x86_64
kernel-2.6.32-358.el6.x86_64
[root@car-lnasdwp-001 /]# rpm -e kernel-devel-2.6.32-504.16.2.el6.x86_64
[root@car-lnasdwp-001 /]# rpm -e kernel-2.6.32-504.16.2.el6.x86_64


NOTE:

You can always revert to the original kernel by setting the grub.conf file back to 0 and reboot as long as you did not remove any of the kernels files for that release.