Is this sort of like what they’ve done in the past with FFX-X2, or are they pretty much remaking a remake at this point?
I don't think it's that extreme, but basically from what I've understood this Ps5 version is fixing the technical problems that the game had on Ps4.
One of the big complaints with the Ps4 release was that there were a lot of textures that the game just refused to load. Thing's like doors, flowers, ground and wall textures, all only loaded their base polygon frames which I think was simply due to the limits on the Ps4 and how it interacted with Unreal 4.
The Ps5 is basically fixing that issue, improving some of the lighting, raising the FPS to 60 instead of a locked 30 (which to the PS4's credit the system did hold a steady 30 almost 100% of the time even on a base model).
So in a way this upgrade is going to be the true experience because everything should work the way the developers wanted it to work originally.
Then there is the Yuffie DLC, which is a full two chapter story with Yuffie that takes place between when Cloud fell into Aerith's church and when the sector 7 plate falls. Additionally they are adding more fights to the optional VR battle arena including what appears to be a new super boss.
The upgrade is free if you already have a digital or disc version of the game and the DLC will cost $20.
Or you get the DLC include for the full $70 if you buy the game new.