anytime you change O or D it sets you back a couple of seasons.  you're better off just recruiting a few good passers here over the next few seasons. if you start to run into huge turnover problems start running slowdown to minimize the number of possessions

if you do decide to change and want to minimize (not eliminate, minimize) the damage, wait til you have a year with 5 or 6 seniors and 8 or 9 upperclassmen, then switch your practice minutes to the new D but continue running the old D in games for that entire season. 

Your vets will have the IQ to get you through the current season without practicing/improving, and the guys who will be returning will get an entire year to work on the new system before you actually switch.  having to bring in the new guys is a wash because they'd be learning a new system regardless.