What is the difference between supersede and replace?

It applies to legal documents between the same parties. When a new document replaces a previous one, it cancels the previous in its entirety, while when it supersedes the previous document, the terms that are unchanged or not mentioned in the new document remain in force as per the previous document, as it to say that "supersedes" means "updates". In practice and for avoidance of doubt lawyers use "supersedes and replaces" in order to cancel the previous document.