Straight from Ubisoft's mouth to your eyes.
Rainbow Six Siege

A short, but succinct tweet from Ubisoft says that Rainbow Six Siege will get a next-gen upgrade for the PlayStation 5 and Xbox Series X. This will be a free upgrade to everyone that owns the game.

Specifically, they say that the game will get a bump in resolution to 4K and a bump in framerate to 120FPS. That is quite the substantial bump on both of those fronts.