not that much different if you dont make everything new
Not exactly true. in UE5 Shadows are handled differently and work much better in all LOD states. So just from the engine switch alone we'll get better looking outdoor areas. Dungeons will probably look about the same though, since they are so dark, but who knows.
Water reflections on the base implementation are improved, so water will possibly look a bit better. This also carries over if they decide to use the UE implementation for rain making surfaces wet and reflective.
And I think UE5 might have raytracing inherently now as well, so with that on and if it runs at ok FPS that will be a pretty good looking improvement for shadows and surface reflections.
Beyond that, it would require work from the SV team to improve aesthetics. As long as new vs old things dont look out of place they shouldn't be in a rush to update older models for new if they decide to use some newer techniques and technology.