Creators will always tell you that they have the right to change whatever they like in their own creations. It’s sort of their privilege – they just don’t do it a lot in retrospect.
Obviously, JK Rowling has the right to change whatever she wants – and boy has she taken that power to its limit thanks to the retconning of Pottermore and other social-media based lore changes – but it’s a different story when directors simply choose to change established details in a film series.
Then again, that’s the price of bringing in different auteurs with different visions. The Prisoner Of Azakaban might be the best film in the series, thanks to Alfonso Cuaron’s direction and creative choices, but it also came at the price of lots of changes to what Chris Columbus had already made.
That’s not entirely a bad thing, of course, but it makes a big old mess of the series’ continuity and it’s a particularly easy thing for fans to pick up on, given how much attention they tend to pay. And there are quite a few instances for them to pick up on.
That’s why so much attention was paid to the decision to introduce the idea of Obscurials in Fantastic Beasts and heavily hint that Ariana Dumbledore is one. That doesn’t cause any real issues, but other changes in the series between movies are far more annoying…