What's the "ideal recommended way to update a .com.zerog.registry.xml file."?
Policy Server 12.8SP2 on RedHat 6
At first glance, the .com.zerog.registry.xml is handled by the installer and configurator and the file should not be modified manually.
The problem of the .com.zerog.registry.xml is that this one has only upgraded version as product value and the upgraded version doesn't exist anymore.
To avoid this issue:
It's possible work around is to get the .com.zerog.registry.xml to the correct version, by installing again the current version in place (1).
So before upgrading, make a backup of the installation and keep a copy of this file. When rolling back, roll back also this file to keep the content in sync with the installation.