WARNING: The following contains spoilers for It: Chapter Two, in theaters now.
It: Chapter Two makes a number of changes in adapting the massive original story for the modern era. As with any adaptation, it's just a necessity in some cases. While the overall narrative is very similar, smaller character traits and beats are modified. The tweaks to characters like Bill (James McAvoy) and Richie (Bill Hader) are big, but they're not as drastic as the those made to Mike (Isaiah Mustafa) and Henry Bowers (Teach Grant).
Mike actually has his role in the narrative increased and expanded, while the threat of Henry Bowers is severely reduced over the course of the movie, to the point where it could have even been cut from the film.
The One Who Stayed Behind
In every version of It, almost all of the Losers' Club have gone their separate ways in adulthood. Only one of them still lives in the town of Derry -- Mike, who's made a life for himself at the local library. As such, he's the only one to retain his memories of Pennywise (Bill Skarsgård), and serves as the one keeping an eye out for his return. When Pennywise does come back 27 years later, Mike is the one who brings everyone back to Derry. The film gives him more to do as the one who stayed behind, though. It's revealed that Mike fully committed to researching Derry and Pennywise. This gave him the chance to learn about the Ritual of Chud, a Native American means of trapping Pennywise.
In the novel and previous miniseries adaptation of It, Mike is removed from the narrative when he's wounded by Henry Bowers. The bully's attack lands him in the hospital, and the rest of the group has to head off to fight Pennywise without him. But rather than removing him from the plot, Mike is merely injured by Bowers in It: Chapter 2. This allows him to accompany the others into the cave Pennywise uses as a base, taking part in the big final battle. He helps set up the ritual, and when that fails he's ultimately the one to realize getting over fears of Pennywise is the only way to kill it. Mike is then fittingly the one to actually pull Pennywise's heart out. All of this gives Mike a significantly fuller character arc than he's had in other versions of the story, and it benefits the character greatly.
Bully Problems
On the other end of the spectrum, Henry Bowers more or less loses his narrative significance in the film. In both the book and the television miniseries, Bowers escaping the mental hospital is treated as more of a threat to the rest of the Losers' Club. But Bowers is severely wounded in his first attempt to hurt the Losers. He targets Eddie first in the new film. This gets him stabbed with his own knife, a wound that proved fatal for him in the original television miniseries. But Bowers escapes and then targets Mike, only managing to superficially injure him before being killed by Richie.
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Removed from It: Chapter Two are the instances where Pennywise actively speaks with Henry as a demented version of a spirit guide. Although Pennywise helps Bowers in the form of a corpse belonging to one of his old friends, he doesn't ever directly talk to Bowers. This takes away from the development of Bowers into a more threatening villain as the story continues, treating him more as a mad dog than a character in his own right. This also means he doesn't get to wound two of the Losers before being barely killed in the attempt. As it stands, Bowers ultimately doesn't impact the story nearly at all in It: Chapter Two.
Win Some, Lose Some
A big part of what made Bowers an imposing figure in the books was how dangerous he was. Especially in the adult half of the story, where Bowers is portrayed as a silent but brutal murderer. Confronting him was almost riskier for the Losers than staring down Pennywise. But by more or less removing the damage he does to Mike and the brief insight into Pennywise speaking with him, he becomes a far less threatening character. He's just a moderate problem that shows up in the movie a few time only to be unceremoniously killed off. If he doesn't have any real impact on the story, then he doesn't need to be in the story at all, and just becomes a distraction in a film that gets close to three hours long.
The exchange, it seems, is the extended development afforded Mike as a result. Not being injured allows him to take part in the climax, as well as see the conclusion of his arc with the Ritual of Chud failing. Mike finally received the level of depth that the majority of the Losers have, and his character is one of the more engaging aspects of the film as a result. Reimagining his role among the Losers gives his storyline more staying power. However, it also means that Bowers doesn't really serve a purpose in the film, and could have easily been cut from the narrative. It's a good trade, as an engaging Mike is a better addition than the loss of Bowers. But the filmmakers probably could have shaved 20 minutes off the film if Bowers just didn't appear in the story as a result, and the movie would be better off for it.
It: Chapter Two stars Bill Skarsgård, James McAvoy, Jessica Chastain, Jay Ryan, Bill Hader, Isaiah Mustafa, James Ransone, Andy Bean, Teach Grant, Jess Weixler, Will Beinbrink, Xavier Dolan, Jaeden Lieberher, Sophia Lillis, Jeremy Ray Taylor, Finn Wolfhard, Chosen Jacobs, Jack Dylan Grazer, Wyatt Oleff and Nicholas Hamilton.