That will either be renewed or not based on multiple factors. In other words, if either party isn't getting as much out of the partnership as they want that will be the end of it.
You do realize that the next Witcher installment is multiple
years away? Like 3-4 years away at best. This is a "we'll be working with them and seeing how it goes, if it goes the way we want, we will keep at it" partnership. Not a "we are forever linked to them" deal. They're testing the grounds so to speak. Like any company would. If development of their first project with UE doesn't pan out the way they want it to, they will move back to RED in an instant. I have no doubt about that.
It's pretty standard commercial partnership fare.
I also have no doubt that UE will
yield the desired results as I firmly believe it's a much better engine for what seems to be their future projects and, as such, they will move on to it permanently eventually. This is just not fact at this point in time and merely speculation from any of us
Meanwhile Epic has probably secured some degree of exclusivity from one of the biggest IP out there. 1 year exclusivity to Epic and GoG is my guess.