Switching to another LLM isn't always about quality. Being able to host something yourself at a lower or equal quality might be preferred due to cost or other reasons; in this case, there's no assumption that the "new" model will have comparable outputs to another LLM's specific prompt style.

In a lot of cases, you can swap models easier but all the prompt tweaking you did originally will probably need to be done again with the new model's black box.

You don't have to, but they will have been optimized for one model. It's unlikely they'll work as well on a different model.

I can't wait for TolkienAPI, where prompts will have to be written in Quenya.