Well, Unity 5 is in beta, and it's generally not hard for a studio to get access to that. But I didn't know the future chapters were coming as patches and not separate clients; that would change things a little bit. And with it still being in beta, I guess it would be a bit unrealistic to expect (though studios coming out with AAA games on unreleased new versions of Unreal is definitely not unheard of, but that's irrelevant)
As for actual porting difficulty, well, it definitely is hard to say without knowing what kind of code RTG has created, but in general, Unity does make it very easy to move your projects up into new versions, especially with how well its object oriented scripting works. If new functions are created to replace old ones, the old ones are still left there working, you are given a warning about them being deprecated and told what replaces it, as well if it doesn't work at all. This is partly a result of the highly cross-platform nature of Unity, most of the changes made to the engine are done under the hood so to speak, as the game code is more generalized and reinterpreted on compile for each target platform. (also, I wouldn't really call that "changing engine", it's the same game engine, just an updated version, nearly all the code should still work).
I do hope they can further optimize though.
Also, something seems funky with the SSAO implementation, it has a pretty huge performance hit even on low, and the differences in visual quality between all 3 levels is almost imperceptible.