The past does not change, but the people permitted to describe it do.

Events are fixed and accounts of them are not. New voices reach the record, new documents surface, and people previously not permitted to describe what happened begin to do so. That is not the past changing, it is the description catching up, and it can feel like revision to anyone who took the older account as the event itself.

When a familiar story is retold differently, ask what evidence or voice is now included that was not before. Sometimes the answer is nothing and it is fashion. Often it is somebody who was there and was not asked, which is a good reason for a story to change.