If you are doing things “properly” then you have good evals that let you test the behaviour of different LLMs and see if they work for your problem.

The vendors have all standardised on OpenAIs API surface - you can use OpenAIs SDK with a number of providers - so switching is very easy. There are also quite a few services that offer this as a service.

The real test is does a different LLM work - hence the need to evals to check.