The answer to this question depends on how you understand the Old and New Testaments. If you see the Old Testament as primarily about God's covenant with Israel, then it would make sense to say that the new covenant has replaced it. But if you see the Old Testament more broadly as revealing God's character and plan for all of humanity, then it's less clear-cut. In either case, though, it's important to remember that God is faithful to his promises, and however we understand his dealings with Israel in light of the new covenant, we can be confident that he will continue to be faithful to us.