Currently you must delete the current image and then select a new one. There isn’t a “Replace”. To be honest, I actually don’t know much about what the developer plans in the future. I just know a bit about how it work and I help out here on the forum. I know that it’s worked the way it does not for as long as I can remember.
I can flag this topic for his attention. He may have more input.
Welcome to the Advanced Custom Fields community forum.
Browse through ideas, snippets of code, questions and answers between fellow ACF users