I think everyone has both “hard” headcanons (things that are so integral to your understanding of a story that you can’t enjoy content that contradicts them), and “soft” headcanons (things that are mutable, where you aren’t strongly attached or you’re open to alternate interpretations) and if you aren’t sure if something is one or the other you can just read a fic where the author disagrees with you and see whether you react with “Huh, that’s interesting” or “I have been overcome with a creeping sense of existential wrongness.”