Retcons are a natural part of comic books, and the history of the X-Men is flooded with them. The identity of Moira MacTaggert, the true sentience of Krakoa, and even Nimrod's history have all been fundamentally changed within just the past decade. Over the course of the X-Men's sixty year history, characters have been regularly updated, shifting entire branches of the team's mythos. The beloved veteran mutant Nightcrawler (aka Kurt Wagner) is an excellent example of this, as his story has changed several times since his introduction in Giant-Size X-Men #1 (by Len Wein and Dave Cockrum).
Source：https://www.cbr.com/x-men-nightcrawler-origin-marvel/
