Preferred form of modification

It's non-deterministic because the training dictates the choices. If you the same LLM gets updated then the behavior of the LLM can possibly change. If you change the LLM then that will also change. Almost always, you will need extra prompting to add more guide rails. This is one of the weaknesses because you never know what the change will do without very extensive testing. You're going to need a lot of expertise here to do that. I reckon that is not going to be cheap.