Retcons are a natural part of any sci-fi movie franchise that goes long enough. Star Wars is a great example, as are many other franchises that use sci-fi concepts like the MCU, of stories that shock viewers in ways big and small by making changes after the fact to alter the course of lore and canon.
Some retcons serve a narrative need to either address a plot hole or fan complaint and get the story back on track. Others are seemingly arbitrary and can have positive or negative effects. Some retcons leave sci-fi franchises in a much worse place than where they started, leading to even more retcons down the road.
The Bad Batch Saved Caleb Dume
A seemingly unnecessary but benign retcon occurred in the first episode of The Bad Batch. The new Disney+ animated series altered the established history of Caleb Dume surviving Order 66 to include Clone Force 99.
It doesn't hurt anything, beyond upsetting the established events of the little-known Kanan Marvel comic book series, and its ultimate impact remains to be seen. By slightly adjusting Dume's backstory, Star Wars has proved that its approach to canon in the Disney era is more malleable than previously thought.
Boba Fett Lived
A much more major retcon occurred in The Mandalorian in season two of the series. Boba Fett was revealed to have survived the Sarlacc Pit, undoing over thirty years of official canon. Comics going back to 1984 had Boba Fett surviving this encounter, so it wasn't a major shock to fans or even an unwelcome one.
It does have major implications for the franchise though. The Book Of Boba Fett will explore his untold backstory post-Sarlacc Pit, something that might just involve other characters from across the franchise who could appear in the Disney+ series, altering canon even more.
Letty Didn't Die
The Fast And Furious franchise didn't start off sci-fi but it's arguably tilted into it in recent installments. Part of that drift is the unlikely return of the character of Letty. Letty is absolutely one of the ten best female characters in the franchise which is likely a big factor in her return after she died in the fourth movie.
She came back in Fast & Furious 6 and has been a major part of the crew since. Still, it was a shock as her fate seemed pretty definitive when she was killed in a car explosion. Real-world logic has loosened considerably in the most recent installments, however, and after a bout of amnesia, Letty is once again cracking jokes and stealing cars in Fast & Furious 9.
Jason Vorhees Is Alive
This major retcon arguably allowed the Friday The 13th franchise to persist beyond the initial installment. The first movie established Jason Vorhees as having drowned as a child. His mother was the killer and impersonated him in a bizarre attempt to avenge his death. The second movie jettisoned this, bringing Jason back to life and setting off a series of deaths and resurrections for the character that occurred over the next 30 years.
Fans of the horror franchise, which went to hell in Jason Goes to Hell and space in Jason X, likely didn't mind this retcon too much as it allowed the series to continue. It was probably a shock for contemporary audiences at the time, though.
Rey Palpatine
Many fans were likely just fine with the revelation in The Rise Of Skywalker that Rey was a Palpatine and the granddaughter of the Emperor. But the retcon opened up some plotholes in the return of Darth Sidious and some complaints from fans who were content with the idea that she had no ties to any established Star Wars character, as established in the previous movie, The Last Jedi. It did lead to some of the most heartbreaking moments in the sequel trilogy, so it's a matter of taste for fans on if the retcon was effective in the long run.
The Mandarin Isn't The Mandarin
One of the most shocking retcons in the MCU occurred in Iron Man 3 when the leader of the Ten Rings, the Mandarin, was revealed to be a nobody. Trevor Slattery wasn't an established character in either the MCU or the comics and he certainly wasn't the fearsome villain fans of Marvel Comics expected.
This shocking plot twist is one thing Iron Man 3 got wrong for many fans. It's likely to be retconned again in the future, as the Ten Rings organization is set to be a major factor in the upcoming movie Shang-Chi and the Legend of the Ten Rings. In that movie, the Mandarin promises to finally appear in the MCU as Shang-Chi's father, Wenwu.
Wanda Maximoff Is A Witch
The Scarlet Witch is one of the most powerful Avengers in the MCU, but her power was established as being divined from the Mind Stone. In WandaVision, her story is retconned to align more closely with that of Marvel Comics.
Wanda is revealed to be a witch whose powers are a mutation largely in place before her exposure to the Mind Stone. Her ability to alter probability and thus reality was retconned to be the reason she and her brother Pietro survived the bombing of their apartment building in Sokovia.
The Force Is Midichlorians
The retcon that the Force is scientifically bound in the number of Midichlorians in a person's body remains a controversial development in Star Wars lore. It demystified the Force dramatically, even as it served to try and explain why Anakin Skywalker was such an unusual figure to the Jedi.
Midichlorians may not have their fans, but they haven't diminished The Phantom Menace's quality as an origin story for Anakin. They also remain a factor in the franchise's storytelling. Midichlorians are at least part of the Imperial Remnant's interest in Grogu in The Mandalorian.
John Connor Was A Terminator
The timeline and continuity of The Terminator franchise are very confused at this point, so it's likely many fans aren't too bothered by any one thing. The shocking retcon of John Connor as a Terminator in Terminator: Genisys was surprising though.
This 2015 attempt at a reboot created an alternate timeline where the adult John Connor was killed and replaced by the T-3000. Many things still hold up in The Terminator franchise, but this particular take wasn't one of them.
Newt And Hicks Died
The direction of the story was uncertain coming out of Aliens, but one thing seemed sure: Ripley, Hicks, and Newt were together. That expectation was dashed with the retcon that Hicks and Newt died in between the movies.
This decision was likely a reason the sequel, Alien 3, didn't go over as well for fans. The unceremonious off-screen deaths of such beloved characters took the story down a dark path it arguably never recovered from, though later retcons tried to salvage what they could of Ripley's story.