Without spoiling the OG?
The OG technically is unchanged. This is some new timeline shyt... Sephiroth showing up to troll Cloud early and Aeris being this attuned so early on implies the FF7 move Advent Children is where this new game picks up from.
Its timeline fukkery
@Brief Keef this ^^^
nothing changes the original whatsoever, but your continued playthrough of the original is probably going to be a lot different to what you will probably play in part 2 of the remake