One of the major changes includes, them being showcased as villains at least in the eyes of Sparrow Academy. The Umbrella Academy can’t just sit peacefully in the new 2019 timeline because at least one of the reasons for coming back has vanished from the timeline.
The entire point of Allison’s heart-breaking decision to pull out of Raymond and come back to the future was that she had her daughter to agonize about. If the Sparrow timeline is set in stone, Claire wouldn’t have existed, so Allison could be the way for sending her siblings back in where they are supposed to be, not least to protect the effect of Raymond’s mislay at the season’s end. However, which mess od The Umbrella Academy lead their father to adopt a new academy is still a mystery.