Which situation would result in a theory being replaced rather than revised?

A theory is replaced rather than revised when the fundamental assertions or ideas behind the theory are proved to be incorrect. When a theory is replaced it is called superceded or obsolete (but only if it was once accepted by the scientific community at large - fringe theories remain fringe theories). One example is Lamarckism in evolutionary theory which was succeeded by Mendel’s ideas of genetics and Darwin’s theory of evolution.