^
I'll be carefull with spoilers, but from the cutscenes I have seen on YouTube:
* Everything between the two bombing missions is new expanded story
* When you meet Aerith you also run into Rude several hours before in the original
* The main goal of Wall Market is the same, but the execution is very different
* When the major thing happens, that show the lengths Shinra will go to to win Wedge survives
The ending I only know from second or third hand comments, but:
* The whispers that show up now and again, are spirits from the future, trying to make sure things happen as they are destined to (eg. happen as in the OG FFVII)
* Sephiroth shows himself to Palmer, a lot sooner, so President Shinra is aware of his presence and is thus not already dead, when the party reaches his office.
* The main Avalanche group (from Before Crisis) attacks the Shinra Building to let the party escape
* Sephiroth kills Barret, but the Spirits revive him, since it wasn't his destined time to die
* Sephiroth tricks the party to destroy the spirits, thus opening up the possibility of changing destiny in the future
tldr; The main story is the same, just expanded. Only the spirits, and their part seems to differ from the original