Update from Redhat 7.3 to CentOS 7.4

I didn't read any documents, but I added CentOS repos and jumped ahead into the upgrade from RHEL 7.3 to CentOS 7.4 and here are the problems I have seen and how I fixed them thanks to the Internet.

First problem:

The update itself didn't work complaining about man pages from PAM i686 conflicting with PAM x86_64.

Solution:

# yum reinstall pam

Second problem:

After the update has completed, when rebooting right after the BIOS finished its job, the system failed to boot, indeed it failed to load GRUB the boot loader with the message:

Failed open \EFI\BOOT\grubx64.efi - Not Found
Failed to load image \EFI\BOOT\grubx64.efi: Not Found
start_image() returned Not Found


Solution:

- Get a bootable USB or CD of RHEL / CentOS 7.3.
- Select "Rescue a Redhat Linux Installation" when presented with the CD / USB options.
 - Ensure you don't get any errors and that you got a message saying that your system has been mounted under /mnt/sysimage.
# chroot /mnt/sysimage
# cd /boot 
- Find your grub.cfg files and *.efi files.
- Copy your most recent grub.cfg file to your CentOS path under EFI dir so that both grub.cfg and grubx64.efi are in the same directory. In my case "centos" dir didn't contain any grub.cfg files:
# cp -a ./grub2/grub.cfg  ./efi/EFI/centos/ 
- Exit twice, either pressing "CTRL + d" two times or type: 
# exit
# exit 
- Remove your CD / USB, and wait for your system to reboot on its own.
- You should be seeing your GRUB menu again and able to boot into one of your kernels. If the most recent kernel doesn't boot for some reason, select the older one and then review your grub.cfg settings carefully for wrong kernel numbers in the same line.