The normal turnover time
Most schools take about three years to adjust fully to a new system.
Year one: New players are recruited while still in high school. The current players only know the old system and struggle in the playing style.
Year two: The original recruits are freshmen and will probably redshirt. There may be five or six players in this group who see playing time. The other players begin to get the new system down, however the older players still struggle.
Year three: The recruits are now sophomores. There may be six or seven starters from this group. The juniors and seniors on the team have now had two years to adjust to the new coaches, so everything is now adjusted. Players who can not adjust, get replaced by coaches recruits.
BYU football is different. Many players who are recruited go on LDS missions. That means that you have to add another two years to the equation. The fact is, most of the players on the current BYU roster were recruited for Mendenhall’s playing style. And they will be for the next two seasons. To give you an idea, there are still some recruits from Mendenhall’s era that are still on their missions and have not even met Coach Sitake.