GitHub Copilot Chat (which is part of Copilot) can change existing code. The UI is that you select some code, then tell it what you want. It returns a diff that you can accept or reject. https://docs.github.com/en/copilot/github-copilot-chat/about...

Copilot will change existing code. (though I find it's often not very good at it) I frequently highlight a section of code that has an issue, press ctrl-i and type something like "/fix SomeError: You did it wrong"