If you upgrade to Modsecurity 2.7.x (RC or higher) your old configuration files will probably stop working with the error message

Rules must have at least id action

The solution is simple, add an id meta-action to the action of ALL your rules as described in the documentation, so start numbering your rules. Yes, previously it was optional and autoassigned and now it's mandatory. 

id
Description: Assigns a unique ID to the rule or chain in which it appears. Starting with ModSecurity 2.7 this action is mandatory and must be numeric.

So this Upgrade needs changing your rules and most likely you will only notice this when the upgraded modsecurity fails to start. You lucky guys out there with many many rules, i guess you will like this new modsecurity version