To edit an existing out of the box Foglight rule. What is the best practice? Would cartridge upgrades break the rule?
The best practice is to make a copy of the rule and edit the rule from there. It is highly discouraged that Foglight users edit out of the box rules as it will affect your default monitoring.
Cartridge upgrades may affect custom rules. Release notes of the cartridge will inform users of deprecated registry variables. Please review release notes before your cartridge upgrade.