I keep seeing the argument among certain fans that it doesn't matter what details Greta Gerwig changes, simply because "all adaptation is ultimately a form of fanfiction".
On the one hand I understand the angle users of this argument think they're coming at this from. Screen is a different medium, the book has to be rewritten (in the case of classic where the author died years ago, almost always) by a different person with their own interpretation of the work, their own bias towards the work, etc...
This alone, however, as a fanfiction writer (initially FOR the Narnia fandom!) of many years, I can 100% attest does not a fanfiction make.
Adaptations when done faithfully (as in taking the themes, characters, setting, etc, as conceived by the author and putting them on screen for a visual audience, not as in some abstract "who can say what faithful really means, since it means different things to different fans" rabbit hole theory) are not transformative enough to constitute what would be allowed on the majority of fanfiction sites without being flagged for copyright. The major change is the medium. Oftentimes if you were to write out the events of a movie in prose and post it (this is called a novelisation btw) you would be asked by most sites to take it down because the work is not transformative enough not to get those who run the site into trouble.
This is also why the trend of going through a book or movie's story with one added character (an OC) and everything else identical to the original work tends to get backlash from fanfic readers. The addition of the one tiny new element might technically just tip the transformation scale to legally allow the story remaining up on whichever site is hosting the fic but it's still a play by play of a story, almost more of an adaptation than a proper fanfic.
Expectations for what makes a good fanfiction (expansion on certain characters, often resetting familiar events in a different time setting or location be it a fantasy realm or a coffee shop, gratuitous descriptions of certain couples being paired up and interacting when they weren't together in the original work) are often the exact opposite of what a fan looks for in a film adaptation of their favorite book or video games or any other mediums that can be adapted.
So when someone criticizes a choice Greta has made (let's say taking the story into the 1950s) and says "it's like she's writing a fanfic", this is, whether you personally mind the change or not, a legitimate argument.
It is common for a Narnia AU fanfic to be set in a different time period just to see what changes with the characters. I had a good friend back in my teenage years of writing Narnia fics who used to primarily write either the Pevensies in modern day or Pevensies in Edwardian era timelines (you know, like Polly and Digory were SUPPOSED to be). But I doubt even she would have wanted the movies billing themselves as adaptations to have done the same. That stuff belongs firmly in fanfic for the most part, given how few (in done cases none) adaptations of Narnia books exist so far.
I'm getting rather tired of seeing the "But what about Clueless!" argument when it comes to this. That is completely apples and oranges.
Firstly Clueless while a retelling of Emma in "modern day" (90s) never presented itself as a straight adaptation of a work with no other screen versions. Cher is not literally Emma. She's a stand in within a modern story inspired by the plot of Emma.
Second, Clueless wasn't made in a vacuum. There was another more traditional Emma movie greenlit/being made around the same time and studios were open to that. People could literally watch either movie by 1996. Unfortunately nowadays you would be hard pressed to find a studio willing to compete with a similar story being made by another studio even if the approaches are vastly different. This is why an allegedly more faithful version of JA's Persuasion was scrapped when the one with the actress from fifty shades with an alcohol problem and a pet rabbit was announced by Netflix.
So if Greta was making "Polly" a 1950s deconstruction of The Magician's Nephew and billing it as a complete reimagining, at worst the naysayers would be disappointed because we don't have a faithful adaptation of MN yet and aren't likely to get one greenlit for years after Greta makes her deconstruction. But I think if it wasn't being billed as a straight up prequel/origin story to Narnia but with a David Bowie Aslan and rock n roll, most fans would have shrugged and moved on. Because THEN it would be more similar to Clueless or Ten Things I Hate About You.
Unfortunately as is it DOES feel like she's using her big name status to just put her million dollar + AU fanfic on screen.
Irregardless of if you view it that way or not, or whether you're personally excited to see the movie, there are going to be a subset of older fans who at the least feel this isn't an on screen translation of the work they've loved for so long.
This does not make half the fanbase "babies" or "idiots who don't know what adaptation means". It just means they're seeing the whole situation differently than those who are excited about this adaptation.