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.