After being very patient, I finally got a chance to play through the game in the last week, so I'll share some quick thoughts:
Overall, I liked the game more than I thought I would, and I appreciate how ballsy they were in wanting to establish new story elements, and not being subtle about their plans to make even more changes going forward.
If some of you are still in denial over this, maybe try sifting through dialogue of the last couple chapters, and how many times characters mention "not bound by fate", "story isn't written" etc.; or maybe the fact that the penultimate boss is literally an arbiter of fate/destiny; or maybe that every character had visions of events from the other games and even Advent Children, and make several mentions about needing to change it; or maybe that the game literally ends with the words: 'The Unknown Journey Will Continue'.
Like it or not, one thing people have to accept is that it's not trying to retread every step of the original, even if it's going down the same general path. I don't mind if they want to keep changing significant moments, but what matters is that they make it feel justified, and somehow find a way to create new significant moments that don't feel contrived. And right now, I can't say I trust them to pull that off.
What I will say is that one chapter at the end of the first game is hardly enough to spell doom for the entire project, and it's a bad overreaction to think so. The next part or two, depending how big they'll be, will be far more important in establishing the new narrative, and whether or not they completely stuffed it.
Worst thing Squenix could do is react badly to some of the fan criticism and try to course-correct by recreating the original events in even more contrived ways. The cat's already out of the bag, and they just have to commit to what they're trying to do while finding the best way to pull it off, because they can't make everyone happy at this point.