If they had the original files surely they would just re-render everything it wouldn't take much time and effort right ?
Probably about the same as time as upscaling the pre-rendered back grounds.
This is another assumption people seem to be making - if they had the files it would be easy to just rerender them.
We also don't know what their tools were, what they were like back then, if they're compatible with modern systems, if they would need to port the tools up to modern systems to work effectively, how long render times on high quality models like their backgrounds are would take, etc etc.
It just seems like people are jumping to a lot of conclusions based off of little to no information on the matter.
They very well could be completely re-rendering the backgrounds out. Maybe they're only doing it for the next gen versions (which could be why it's coming out later) in order to ensure that they look visually superior as people expect with next gen. I have no idea, but the point is there are a lot of different possibilities.
The current gen release in Japan isn't until November 27th, the next gen release isn't until next year, and the PC release isn't until later next year.
If they do have the original files, and they can rerender them, and it is a quick process to do so with modern hardware, it is possible that the work they've done up to now is simply getting the game engine working perfectly and the next 3 months will be spent rendering and replacing textures. Or they could not. The point is we don't know.