It can be done. You can make a movie that gets the spirit of the book while leaving little of it strictly intact. It helps when the source material is essentially a very dark fairy tale, and fairy tales can be be reinterpreted quite a bit without losing their essence. It (2017) isn't a bad example so far, though I'm leery of things I've heard about chapter 2, particularly concerning Mike.
I think it was the right choice to bring it forward in time. The book was never meant to be a period piece -- it was the past and the present in 1985, and now it's going to be the past and the present in 2019. Because of the nature of the monster's manifestations, moving it up to 1989 when the Losers' Club faces It means that the pop culture references would certainly be different, so the visions would be different. In 1989, you were pretty unlikely to have been moved to terror by, say, I Was a Teenage Werewolf or Rodan, which are two of the forms It takes in the books. Beverly would certainly not have been as naive about her father's intentions in 1989 as she was in 1958. Richie wouldn't have been cracking the same jokes, and they wouldn't have gotten away with damming up the Kenduskeag in the Barrens. So several perfectly logical changes flowed from the initial change.
Some things were neat additions -- Bev's hair coming up to strangle her, the vision of Betty Ripsom, Richie's "Now I have to kill this fucking clown" line.
Some things were wise omissions, like the sewer scene, WE ALL KNOW WHICH ONE. That was questionable in 1985 when the novel came out, and 1985 was more, um... well... how to put it? Let's leave it at the idea that, at the time, it was a lot less over the line than it is now, but it was still over it. Replacing it with two innocent kisses and some staring from the boys was a good choice -- the book was a lot about awakening sexuality (in Bev's point of view, she even points out that sex is called "It"), but it didn't need that.
The setting looked much more like a Stephen King setting than the old movie did, or like most of the adaptations do.
Some choices were poor. Having It kidnap Beverly was a bad choice. For one thing, why turn her into a damsel in distress? For another, the focus on Bev took a good deal of focus from the others, making her seem more important than they were. Bill is meant to the be the hero of the story.
I cannot figure out why Ben got the Derry historian role. Yes, Ben liked the library in the book, but the historian is Mike. He's also the one who ends up a librarian; Ben's interest is in buildings and architecture. It's after he joins the group -- he's the last, because he doesn't go to school with them -- that they get the rundown on the place. There's not a good reason to make that change. Ben already had the crushing-on-Bev storyline, and Mike barely got one at all. The focus on Stan's Jewishness was interesting, though a little overboard (and, as was pointed out on TVTropes, it was ridiculous that his supposed rabbi father -- not a creature of the books, where the Urisis were pretty secular -- complained that he wasn't getting his Torah reading right while he was practicing his blessings, not a Torah portion). I missed the real quirk of Stan's character, though, which was that he was a neat, ordered little boy who wasn't so much frightened of It as an eldritch abomination, but offended at how Its existence offended the order of the world and made things stop making sense. It was Stan, not Bill, who once defeated It in the form of a bird by saying he had memorized all the birds and this one wasn't real at all. He also said that he could stand being scared, but he hated being dirty. Those things were more interesting, I think, than what the filmmakers did.
I know it's a lot to ask, cramming the whole childhood story into two hours, but I did feel like some of the Losers really got the short shrift, and we weren't allowed to know them and how they related. A lot of Richie's part is cut, leaving only his line that Bill didn't stutter on a speech as a kind of out-there sign of the fact that his brain works a lot faster than most people's, and he understands Bill better than any of the others do. That's why it kind of comes out of left field when he's the first to jump into the final fight.
The problem is that the book is exactly the length it needed to be to accomplish what it accomplished. (Well, except for two paragraphs in Tom Rogan's point of view which literally repeat each other word for word EDITORS.) You could never get it all into a theatrical cut. Now that they can make bingeable television which basically turns into ten hour movies that don't have to worry about network rules, I'd have gone that route. You could even have "breather episodes" doing the Derry interludes, and man, would it be sweet to see Dick Hallorann in the fire at the Black Spot. That is the sort of thing you cut for a two-hour movie when you're trying to keep the story focused, but losing it leaves only the bare bones of the story, and the story's bare bones are like any other monster vs. kids story.
Of course, probably some things that should have been changed weren't. I never met a Beverly of that age, or an Elizabeth called Betty. I did know a single George my age, but it was a family name. I knew of a Henry. Those were all 50s names. One would be one thing -- like the George I knew, it could have been a family thing. But all that age and no Jenny or Chris or Jeff or Lisa or Kim... but still Beverlys and Bettys and Henrys and Georges? That's where it gets odd.
Anyway, verdict: Not awful. A decent enough updating, with changes that didn't destroy the thing. Good kid acting (are we getting a crop of directors who actually know how to direct child actors?). But ultimately, too much of the heart of the novel had to be cut for time.