After trying several paths, some suggested on this list, here's my results.
1) Fixing a unbootable system wasn't practical in my case. Fortunately, all my systems can be rebuilt from scratch.
2) When I was lucky enough to catch an updated system before reboot, backing out the defective updates wasn't possible. Yum said there were no prior versions.
3) The most reliable method I found for Centos 7 was: - Re=install from scratch (luckily, my data files were safe and restorable) - Before running any updates, apply the fix suggested by Redhat and exclude updates to grub2, shim and mokutil. - Without the above 'exclude', the system became unbootable after a yum update even though the corrected versions of shim should have been loaded.
The system I'm dealing with is Centos 7. I can easily rebuild it from scratch and test stuff without losing crucial data, if it would helpful.
4) I haven't experimented yet with centos 8 because the hardware is remote and requires me to get a friend involved to help. My local hardware is not supported by Centos 8, so it will remain on Centos 7 until I replace the hardware or switch to a different Linux.
David