If you choose to implement branching logic in your rule sets, then you will first need to identify which rule attribute implementations will need to have branching logic. (Indeed, if this identification shows that a very large number of attributes require branching logic, you may be better off switching to the Multiple Product Periods for Your Product approach instead.)
For each rule attribute that requires branching logic, use CER's legislationChange expression to implement different logic for different "eras" of time. See legislationChange for a full description of this expression.
Update your rule set tests to test for the behavioral changes introduced by your legislation change.
Once your changes to your existing rule sets are complete, publish the changes, which will result in existing cases being bulk-reassessed, typically leading to new determinations being stored for some existing cases.