I've never really had to do a retcon. I feel like retcons are more a thing that happens when either you've taken over a series originally written or run by somebody else and want to insert something into older continuity they didn't think to put in there to make your new story work, or if it's your own series, but you're making a sequel or prequel you never really planned for when you made the original.
When I felt frustrated and painted into a corner with my old series, Fan Dan Go, because it wasn't particularly popular or making significant money, I didn't need to retcon, because I had the option of rebooting. Errant is a hard reboot of Fan Dan Go where I was able to make a lot of changes that would have been awkward to retcon, and I was very careful when creating Errant to plot things out in advance so I wouldn't need to retcon things in. If you go back and read the prologue again after getting up to date, you can see I planned a long way ahead, like how Jules is in the prologue, despite not appearing again for a couple of chapters, and even the fact that Jules finds Sarin attractive is foreshadowed there.
So if I can avoid having retcons at all, I will. I'll always try to make sure a series is planned ahead for the length I plan for it to run, and keep some of the setting vague enough to add detail. I hope I never need to retcon anything, honestly!
...But if I did... I'd probably try my best to make it as unintrusive as possible. If I could only contradict things the audience had been told rather than actually seen, it could at least be passed off as misinformation. To use a Metal Gear example, there's a character who's a doctor who in earlier games is not seen, only talked about, and in Japanese, the character is just referred to as "sensei" "-san" etc. while in English, the translators had to pick some pronouns (because English doesn't really work without them) and they picked "he/him" when talking about this character...who then turned up in a later game and was a woman (probably not planned ahead, but in Japanese that doesn't matter because the language barely uses gendered pronouns), so the English version of MGS4 had to add in a conversation where Snake was confused and he was told that there'd been some misinformation saying the doctor was a man. A little clunky, but believable enough!
When the audience has seen it though... that's where it gets hard. Like was it an illusion? Was the character hallucinating? Implanted with fake memories? A disguise? I'd avoid this at all costs because I really, REALLY hate that one scene in the game "Heavy Rain" where it's revealed who the killer is, and it's somebody who you didn't suspect because you literally played through their alibi as them, and then the game is like "ahaaa! But that didn't really happen!" and I'm just there like "RRRRGH! WHAT THE F-!" That's just lazy, it's cheating. It's punishing people for caring and paying attention. If I can avoid ever doing that, I will. I'd rather have a character change their mind or make a new friend than try to shoehorn in "yes, actually they've always thought this" or "They've always known this person, they were just...uh...never around?"
