1) Cutting players who are buried on the depth chart - Do you allow players who likely will never contribute languish on the bench for years using up a scholarship or let them loose to pursue playing time elsewhere? This is one of the hard aspects of running a program, but letting the players you misevaluated go could be a win-win. Some players that decided to stay at BYU have been given scholarships, but not as a football player.
2) Asking players to delay enrollment a semester - This is not ideal, but the player still gets the 4 years of scholarship once he enrolls full-time. If the player doesn't like it, BYU risks losing the player.