Warning! SPOILERS for IT Chapter Two.
The story of the Losers' Club and Pennywise may be continuing in IT Chapter 2, but the new movie also retcons the 2017 original. The first IT served as a largely standalone story as the kids of Derry fought back against the evil force terrorizing their town and killing children. But the movie also clearly set up that IT Chapter Two, with the kids going their separate ways and Pennywise retreating for the next 27 years.
When IT Chapter 2 begins, it quickly begins introducing the grown-up versions of the Losers' Club. Once Pennywise attacks again, Mike Hanlon (Isaiah Mustafa) calls each of his former friends up so they can make good on the oath they made 27 years ago. This allows IT Chapter 2's cast to feature more recognizable, established faces, including Jessica Chastain as Beverly Marsh, James McAvoy as Bill Denbrough, and Bill Hader as Richie Tozier. However, IT Chapter 2 also uses flashbacks to keep the young cast involved. But although that's a narrative device out of Stephen King's book, it nevertheless leads to a minor retcon.
The retcon comes with a repeated focus of the flashbacks on the time during the summer of 1989 when the Losers' Club split up after an attempt to defeat Pennywise in the Well House left Eddie with a broken arm. IT Chapter 1 made it appear as if this split didn't last very long and was rather uneventful for each of the kids. But in IT Chapter 2, the flashbacks explore how Pennywise tormented each of the kids individually throughout the summer, lengthening the period and adding a lot more trauma.
This retcon may not be a bad thing - it allows more Pennywise, for one - but it does reframe the overall story. The first IT just showed Bev and Bill bored at their respective houses, Ben back at the library, Richie attending Stanley's bar mitzvah, and Mike using a captive bolt pistol. When they all eventually regroup, there is no mention of further run-ins with IT. As Chapter 2 shows, however, Pennywise transformed into a lumberjack to scare Richie, pretended to be Bev and burn her head to scare Ben, makes Eddie believe his mom was kidnapped and becomes the lepper again, and much more. Perhaps most strikingly, it's in these flashbacks that the first seeds of Richie being gay are first sown (a plot twist barely hinted at in the original film).
This retcon is all the stranger given that director Andy Muschietti and the studio knew they were going to make IT Chapter 2 while shooting the first movie. Of course, there is a clear explanation for why IT Chapter 2 added more adventures for the kids that weren't mentioned before: the popularity of the child actors. While pre-release only Finn Wolfhard could be described as a recognizable face due to his part in Stranger Things, the group were a breakout hit following IT's box office smash. It would have been illogical to not include as much of the young cast as possible in the sequel.
While this IT retcon may stand out right now, it may be smoothed out by subsequent re-edits of the film. Muscietti has teased that there have been talks of combining both parts of IT into one five-hour movie that would restructure the story. If that cut does happen, then maybe the director will manage to eliminate the feeling that the added adventures in IT Chapter 2 are a retcon.