Revisionism is a fundamental alteration of a theory, essentially usurping (though taking elements of) the former theory and replacing it with a new one. While the attributes of a theory are subject to change in accordance to changing historic circumstances, changing the fundamental basis of that theory is to nullify it in place of a new one.
Eduard Berstein, infamously noted in Lenin's "What is to be done", was a leading theoretician of the German Social Democratic Party. Originating the revisionist theory of evolutionary socialism. He rejected Marxist concepts of Class struggle and world revolution, liquifying Marxism. Removing Its revolutionary elements to focus entirely on Reformism.