Yeah, but there's "Dumbledore was gay" level retconning, where it wasn't ever explicitly stated in the books but looking back there is some evidence in support of it and it's believable that JKR had that in mind the whole time but was being subtle about it, and then there's "gratuitous retconning to try and make things more interesting." The Obscurus/Obscurial falls into the former category IMO because it makes so much sense that Ariana Dumbledore was one and that that caused the rift with Dumbledore/Grindelwald and Grindelwald's obsession with them following that. It still could end up in the latter category if they really butcher the way they play into the plot in the later movies.

JKR has been criticized in the past for trying to retcon in diversity (like her suggestion that Hermione might be black) just for the sake of staying relevant and I'm not particularly keen on that. Right now, I don't really think there's any evidence in the books to suggest Nagini was more than just a snake... so we'll have to see whether the movie makes her humanity relevant at all or whether this is just a way to shoehorn in another character with a tie to the main HP timeline.