A work is a sequel or prequel to a work that is out dated in one way or another but subsequent works have to keep them. Why? Because it's canon. One way of explaining this is that the work then becomes an Alternate History set in an Alternate Universe. See also: Zeerust, The Great Politics Mess-Up, Cosmetically Advanced Prequel, Two Decades Behind. And Retcon, for when this trope is averted. Examples of Zeerust Canon include: