If BYU is 1-4 and Mangum is really struggling. If they get out of Sept at 2-3 or better, I think they stick with Mangum. I really think they would prefer to get Wilson some experience but also still be able to redshirt him.
Ideally you would like Mangum to go out with a good senior year and Wilson to get 4 games where he gets a decent amount of playing time, get some experience under him and still have 4 more years to play at BYU