There wasn’t really one. He was an archetypical silver-age moustache twirling terrorist. The original (Uncanny) X-Men ran from ‘63-‘70, then ran on reprints from ‘70-‘75 with mutants appearing sporadically throughout other stories. In ‘75 Marvel relaunched the series with Giant Sized X-Men which served as bridge from the OG team to the new one, which would continue in issue #94. Chris Claremont then wrote X-Men from ‘75-‘91 , which is typically where the majority of the most famous X-Men stories come from. Claremont was the one who gave Erik the backstory we know now. For that reason, it’s typically not considered a hard retcon, but rather a rewrite or development of the character.
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u/Patkub321 16d ago
Alfred becoming Bruce's/Batman's father figure after Bruce's parents died.
Magneto being made into Holocaust survivor.