When is a new contract replacing an old on enforceable?

A new contract replacing an old one becomes enforceable when both parties agree to the terms and conditions, demonstrating mutual consent. Additionally, the new contract must contain legal consideration, meaning something of value is exchanged. It should also meet any legal requirements, such as being in writing if mandated by law, and must not violate any existing laws. Once these criteria are satisfied, the new contract supersedes the old one.