The past is partly known and completely immutable. Our understanding of World War II can change but not the fact that there was a World War II with certain dates, certain major events and turning points and many details, known or unknown. It is conceivable that someone might seem to himself to have been transported from 2020 to 1940, thus to be living among the events of World War II. However, if he makes the events of 1940-1945 different from the way that he remembers they were recorded, then, wherever or whenever he is, he is not in the World War II that is part of our known history. There must be some other explanation of his experiences.
All this has been discussed before, of course, but maybe we can clarify a principle? The proposition that the past cannot be changed applies not only to our familiar experience of time but also to any time travel scenario. Travel through time would involve travel to the past and the past cannot be changed. Anyone who, e.g., assassinates Hitler in 1940 is not in our past, therefore has not time traveled.
