Warning: Spoilers ahead for IT Chapter Two.
IT seemed to set up a role switch for Mike Hanlon and Ben Hanscom based on the Stephen King novel, but IT Chapter Two retconned the setup. Mike and Ben were portrayed by Chosen Jacobs and Jeremy Ray Taylor, respectively, in the first installment by Andy Muschietti. Actors Isaiah Mustafa and Jay Ryan took over as the adult versions of the characters for IT Chapter Two.
In King's 1986 novel, young Mike was the last kid to join the Losers' Club. He introduced an old photograph album to his new friends, revealing some of Derry's history, including IT's presence spanning decades. Mike served as the historian of the group, but that role went to Ben in the 2017 movie adaptation. Ben, the new kid in town, spent a lot of time at the Derry local library. After Pennywise started to torture Ben and the fellow Losers, Ben did some important research on the town.
Mike and Ben's roles seemed to be swapped heading into the sequel, but IT Chapter Two strangely ignored their past and reverted back to Mike as the historian. Rather than keep Ben as the researcher and the hunter of logic as the first film set up, he followed the book narrative and became an architect. Mike, the once Derry outcast, spent the next 27 years living in Derry where he became a librarian. He also spent much of his time tracking Pennywise's origin, as well as a way to defeat the entity for good. So what was the reason for the odd switchback?
It's easy to question why IT didn't just set it up as Mike being the historian from when they were kids. This would have made it more fitting that he remained in Derry awaiting Pennywise's return. What's even more strange is the fact that an IT Chapter Two flashback showed that Ben was building the Clubhouse in the Barrens in the early timeline. So the sequel ignored his time spent in the library to provide foundation into his future as an architect.
More than likely, Muschietti and the writers wanted to give Mike more to do in IT Chapter Two. Young Mike got pushed aside for most of the first film. By giving the character his arc from the book, he played a much more crucial role in the sequel. Adult Ben already had a significant subplot with his unwavering feelings for Beverly; there would have been no way to equally portray Ben as the historian while also teasing his romance with her.
The portrayals of the older Mike and Ben characters weren't the only arcs that got adjustments between the first film and IT Chapter Two. There were no connections in the 2017 film that indicated that Richie was gay. To back up that twist, the backstory was added in the sequel through flashbacks. It's interesting that those clues weren't highlighted during the first movie. The same goes for Mike and Ben's respective fates. It's possble that the connections and arcs will be cleared up if Muschietti ever combines the two parts for a special cut of the films.