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.
Revisionist theory, politics and praxis are only so when examined thru the lens of dialectical materialism and found wanting in one of two ways. Contradicting past successes. The vanguard party works thanks to its application in the historical successes of the Bolsheviks, Vietnamese Communist Party, and the Communist Party of China. Or by pushing errors with a history of failure and incoherence. The narodniks were unable to cohesively grasp that the feudal landlord ends with the capital production of agriculture.
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.