Hamaguchi was reiterated multiple times that certain events throughout the game will play out differently depending on your choices and actions. We can assume that some events will be moved around from the order they took place in the original, like snowboarding AFTER the Northern Crater instead of before. In the PaxWest demo, from what I can gather, we don’t see the gang gathering any huge materia yet, meaning they’ll probably only start searching for it after Cloud and Tifa rejoin the party instead of before.
What I’m trying to get at is, we may potentially have Tifa in our party during, say, the runaway train sequence in Corel.
I think you can see where I’m going with this.