The official story is that they changed the rendering system because one looked nicer during the day and night and the old rendering system (the one that allowed volumetric lighting) somehow didn't look as nice at certain times of the day. But if you followed the development closely, you'd also know that prior to E3 2014 demo, Velen and Novigrad were two separate regions. When you know all the details it becomes obvious that the change in the rendering system didn't happen because the game didn't look nice during certain times of the day. That's nonsense. We've seen gameplay footage and screenshots taken during various times of the day. The rendering system was changed (downgraded) because consoles couldn't handle the huge chunk of the world that is Velen+Novigrad with graphics that were as nice as shown in the Sword of Destiny trailer, and they were aiming for parity.
I don't actually mind the downgrade. It's still a beautiful game and I think they made the right call unifying those two regions into a single entity. But we shouldn't be blind to the fact that CDPR wasn't honest about what they did.