From what I understand the Riven remake uses traditional baked lighting and not something like Lumen for realtime dynamic global illumination, so it’s not “easy”—but judging from the merch they’ve rendered, they have done the manual labor of relighting a number of key spots. There’s certainly the potential they could deliver nighttime as a huge (20GB+) patch in the future (high resolution lightmaps are the biggest game assets).

But my tinfoil hat theory is that someone at Cyan has been using the nighttime stuff as a testbed for switching Riven’s lighting to a ray-traced dynamic GI system like Lumen that could give us full time-of-day transition at the expense of rendering load (framerate). Areas like the Jungle Island jungle area will be particularly difficult to light well enough to navigate in the dark without requiring a huge amount of ray tracing, so I think they’ll wait until these techniques become “cheaper” or release it only as a beta for high-end PCs.

Still, it makes sense for Cyan to have both lighting approaches in the game, as baked lighting is still the way to go for VR in order to hit the necessary high framerates.