the last two years I went searching for any studies on the effects of changing coaches in college football. Here is an interesting study I found.
http://freakonomics.com/2012/12/21/is-changing-the-coach-really-the-answer/
Basically the conclusion is that if your team sucks changing coaches has no effect. But if your team is middle of the road (which I would argue BYU has been closer too) changing coaches more often leads to worse performance instead of improved performance.