That said, I wonder what is in it for them to make this Witcher 3 upgrade at this point. I mean, surely by now everyone who wants Witcher 3 has bought it already. And if they make it a free upgrade, then we are looking at lots of cost to update it, and not much added revenue.
I can think of a few good reasons - these aren't necessarily exclusive of each other, and I won't claim to have any extraordinary insight, this is just me guessing.
CDPR is known for doing this kind of thing - providing free updates, free DLC, etc. So it's not out of character with them, and it builds a lot of good will - that goes a long way when things like the W3 trailer scandal
broke, or the hot mess that CP2077 was.
There is a good chance that a lot of folks that work at CDPR just really like Witcher. They've made 3 of them already, it kinda put them on the map, and I bet a lot of people have put their hearts and souls into the game. So for them, maybe it's just because they want to.
It could also be a tech demo type thing - where they are just updating their RedEngine with some new toys, and using a very stable and well-debugged platform in order to test the capabilities in the engine. IDK about this one, because CP2077 had a lot of these toys already, but ... it kinda released as a ****show, so maybe the combination of new game + new toys was too much.
Now that W4 has been announced as being released on Unreal, maybe there isn't too much to that last one, but it might also just be a way for the devs to figure out how to use some of these toys in a more familiar environment before trying to push too far in Unreal.