The new contract only partially replaces the old contract, where no new clause is specified the old still stands.
After all, God is constant. His beliefs don't change, if he ruled against something in ancient times then that still stands. Except in those parts that he felt the need to update. Or which everybody ignores now days because some parts turned out to be impossible to keep and others are just silly.
For certain Christian sects. One of the more subtle but important differences between different Christian churches is exactly how they view the relation between the old and new.