If you choose to use multiple product periods for your product, then typically it will be helpful to clone the rule sets for your existing product period (assuming that the structure of the rules is still suitable, and that some significant number of existing rules implementations do not require updating to implement the legislation change).
Clone the rule sets using the process described above in Cloning CER Rule Sets. You need only clone the rule sets down as far as those rules which are affected by the legislation change. It may be helpful to rename your rule sets in line with your eras of legislation. You may also identify useful refactorings to make common any rule classes which are unaffected by the legislation change.
Update your cloned rule sets to implement the change in legislation.
You must create tests for your rule set changes; because you have cloned rule sets, you may need to clone your bank of rule set tests, and update the cloned tests to use your cloned rule sets. This cloning of tests will help ensure that existing functionality unaffected by the legislation change continues to work as expected. Update your cloned rule set tests to test for the behavioral changes introduced by your legislation change.
Once your changes to the newly-created cloned rule sets change are complete, publish the changes.
Set an end date on the existing product period. Create a new product period (see Write the Product Periods) and configure the new product period to use your cloned rule sets.
Publish your changes to the product periods, which will result in existing cases being bulk-reassessed, typically leading to new determinations being stored for some existing cases.