alt Hacker NewsAll the model companies except kind of Anthropic (and even they half-assedly do) implement the OpenAI API. It's not an open standard but, like the S3 API, it effectively is.
And, to answer your question, no. The existence of a common API makes it trivial to change zero code and send requests to a different model.
The Anthropic messages API is a competing standard (it's just better than the OpenAI API which even OpenAI has moved away from) and some Chinese providers use it as their standard.