how do we migrate our rental actors? is it an actual code change or just monetization setting change?

It depends if you can migrate with default events (default dataset item and Actor start) or you want to charge custom events For the start, we do recommend to use default events. They don't need any code changes. Actor start is charged automatically when set once per GB of memory Actor uses and default dataset item is charged once per every data that is stored to Actor's DEFAULT dataset. If you need anything else, you will need code changes - and starting to charge the event manually in code, as per documentation

I would like to avoid making code changes so default events sounds good. Does that mean I can wait for them to automatically be converted and then add pricing later on?