You have OpenNMS installed and configured it to your needs. Now there is a new major version and you want to upgrade. On major versions configuration file schemas can change and your custom changes need to be migrated to a new version.
You only have to migrate what you really have changed, right? The diff command is available on every modern Linux distribution and we can use it to identify these files.
List files we have added and do not exist in a pristine install
cd /opt/opennms
diff -rq -EBbw ./etc share/etc-pristine | grep "Only in ./etc[/|:]"
List files we have changed and need to be merged manually
cd /opt/opennms
diff -rq -EBbw ./etc share/etc-pristine | grep "differ"
List files we have deleted and exist in a pristine install
cd /opt/opennms
diff -rq -EBbw ./etc share/etc-pristine | grep "Only in share/etc-pristine[/|:]"