As long as it is still the same basic story and keeps all the best bits and characters intact, then it doesn't matter too much that Bob's bald, Alice dies in a train wreck instead of a car crash, the football game ended with a different score, and they cut the watermelon scene, right? It's a bit of a shame they screwed that bit up, but really, it's not as if the entire work is Ruined FOREVER, right?
...or so you would be told by many, many, many fans.
For some people, the very act of adaptation is decay. A film version of something should be a direct word-for-word transcription, with utmost care that the sets, costumes and people be reproduced in every detail. If a character who wears a homburg in the original now wears a fedora, that will be enough to ruin the character, and therefore ruin the film. It will be all you will hear about from these fans on message boards, with them going on at length to explain how his homburg visually defined his entire personality in a way that a fedora never could (even though both look almost exactly the same, except for the top).
And don't you dare suggest that in changing it they made it better. If certain fans take the Chicken Little approach and announce that the sky is falling, it's mainly an expression of their fears that the writing staff don't care much about the source material (particularly if it's not mainstream-friendly).
This also happens a lot with translations. People can become religiously attached (sometimes quite literally, in the case of texts like The Bible) to one translation of a work, and when a new translation comes out they condemn it as a travesty, accusing it of distorting and cheapening the meaning of the original, whether or not the new translation is a more literally accurate rendering.
This trope is not only used for adaptations and translations. It is also applicable to ongoing series where a significant change is made between seasons. The trope can be explained in terms of prospect theory, where fans are much more averse to losing any aspects of the original, compared to the enjoyment gained from any "improvements".
Sometimes, the complaints are about a sequel or remake to an adaptation, and the original adaptation actually made changes from its source material, and the new adaptation tries to be more faithful, but people don't know that and complain about changes anyway.
This is sometimes a result of Ability over Appearance.
But sometimes the fans who complain may be right. When producers make drastic changes just for the heck of it, it can get rid of any redeeming qualities the property had. An established franchise making sudden, drastic changes usually heralds the beginning of Running the Asylum and/or a Dork Age. If fans liked the franchise for what it was, and then it is totally changed, the new stuff may be good and please the fans, but they are right to complain that what they liked is never coming back. An adaptation that goes through too many changes can become a bastardization of the source material that seems like an insult to the fans, or even become In Name Only, fans can even argue about why adapting something popular to profit on the existing demographic, but don't deliver something that connects to them. And the most important, they can argue that they aren't complaining because it was changed, they are complaining because it was changed for the worse, the new version is not just different, it is bad. The introduction of online updates to video games can make the criticisms more valid: unlike a new sequel that can simply be ignored, updates can effectively make the game they enjoyed non-existent. Nerfs are particularly likely to cause fan uproar.
Compare Translation Style Choices, Darker and Edgier, Lighter and Softer, New Sound Album, Replacement Scrappy, Ruined FOREVER, Seasonal Rot, Network Decay, Jumping the Shark, Fanon Discontinuity, They Don't Make Them Like They Used To, I Liked It Better When It Sucked, or even Damn You, Muscle Memory!.
Remember that Examples Are Not Recent, even if you're writing up an example while fan controversy is erupting! Remember that some of us will be reading these long after the fact.
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