IT: Chapter Two was a relatively faithful adaptation to Stephen King's book, just as the first one was, but there were a few major changes that altered the course and lore of the film.
Though it was a smash hit and both films have done tremendously well at the box office and are appreciated by fans of King's novel, IT, the changes that director Andy Muschietti made were highly talked about after the film's initial release. Some debated whether his changes made the overall story better or worse. There seems to be a mixed bag of responses in that regard, but for the most part, people have a great deal of respect for the source material as King's novel remains one of his most iconic and popular works.
IT: Chapter Two had a few minor changes, but three major ones stood out as they changed not only the story, but the scope and lore of the universe.
Stan Uris Didn't Leave Any Letters
In some ways, the film's biggest and most emotional change was the one regarding how the Losers Club dealt with the suicide of their friend, Stanley Uris. Stan (Andy Bean) reacts in a similar fashion to all the other Losers once he receives the phone call from Mike Hanlon (Isaiah Mustafa) that implores him to return to Derry like they promised, since he believes Pennywise has returned. The other Losers are equally terrified at the prospect of another battle, especially 27 years later when they are older. Stan makes the decision, immediately following the call, to commit suicide. His wife, Patricia, finds him in the bathtub with "IT" painted in his own blood.
In the book, this is the only explanation given for his suicide. Stan's story is primarily told through his wife, Patricia's narration, and the Losers never get any kind of closure from his death. In the movie, Stan writes each Loser - as well as his wife - a letter explaining why he did what he did. He was too scared to face IT again, but knew he would have to go otherwise, and figured he would be their weak link and could, potentially, lead them all to their demise. Stan "took himself off the board."
The Deadlights Give Bev Visions
The Deadlights are a major part of not only IT but the Stephen King universe in general, having ties to The Shining and The Dark Tower. In IT: Chapter One, Beverly (Sophia Lillis) ends up face-to-face with the Deadlights, which shine into her eyes and put her in a trance. According to the book, an encounter like this with the Deadlights will either kill someone or drive them insane. She is saved from the Deadlights by Ben's (Jeremy Ray Taylor) kiss, which revives her. As an adult, Beverly (Jessica Chastain) ends up being haunted by visions of her friends' deaths during her time away from Derry. She doesn't remember these visions until she is reunited with the Losers Club. IT: Chapter Two comments on her visions and even uses them as a primary plot point while the Losers prepare for their final battle. Beverly doesn't have visions in the book, and never looked into the Deadlights at all.
The Movie Changed The Ritual of Chüd
The Ritual of Chüd is what will kill IT for good. Mike has spent years researching how to kill IT, since he was the only Loser who stayed in Derry. The Ritual of Chüd requires the Losers to collect tokens from their childhood and sacrifice them to trap Pennywise in a vase that Mike stole from a local Native American tribe. In the movie, the ritual exposes the Deadlights and each Loser has to go through their own, individual attack on Pennywise before fighting him together, which they do by taunting him and making him "feel small." Since Pennywise feeds on fear, the Losers deduce bullying him effective instead, which it is, and they crush his heart. In the book, the Ritual of Chüd is what actually kills him. The Losers Club perform the ritual once as children and again as adults. In the movies, they only learn about the ritual as adults.