The game is already "next gen" in terms of how you plebs understand it.
It already runs in 4k, it already has ray tracing and it already has all the niceties you really don't understand.
The issue is more making sure it's balanced for the new machines. They hit a point where they had to make sure it runs as well as it can on the Boner without being embarrassing, as much as it needs to run on the X/PS5 and be flashy. The code, the graphics, the game are all there, they just need time to make sure it is tailored to work on them properly.
In Layman's terms, it's not so much about "updating" the game as it is about testing it to be the best it can be on the new machines.