There’s a couple of ways to approach this. One, (if you didn’t do it this way) open a Copy of your 4.6 project in 4.9 without overwriting the original. Or two, create a new 4.9 project and migrate your assets. I feel your assets may be too large, as I stated before, but if you feel you will have less problems, in your case, with Unity, then it’s understandle if you make the switch.
Hopefully, though, you will be able to take advantage of UE4’s other powerful features in future projects.
Thanks for the feedback.