Oh they're 100000% changing shit. I'm still skeptical that they're going to make some HUGE changes (like a certain thing that happens in a certain city named after a race of people that are nearly extinct - I'm 99.9% sure thats still going to happen) but we're well beyond a 1 to 1 retelling of the original story.
Like if Zach shows up in a chariot and saves the day at said city, I won't even be able to finish the game but with all thats out there, I'm sure that isn't happening.
I think the call of meteor and the "rebirth" of a certain guy+ the summoning of the weapons is an easy end point if we're just looking at it from a narrative perspective, but there is SO MUCH GAME between leaving Midgar and that point. You can totally move some things around (e.g: make Wutai exclusively a pt 3 thing) but you'd literally cover the entire map aside from Wutai and Mideel by going in this direction. Lots of areas and stuff you can revisit and thats before getting into wonky timeline stuff, but it kind of seems like pt 2 would be overstuffed and pt 3 would be really light.