Well, I see it a bit differently ..... Both Ash and Fickell will be sitting down prior to speaking with their assistants on defense and run the systems by Meyer (you can bank on it). I think Ash will have more to say about the back end of the defense, while Luke will be responsible for anything close to the LOS. This will not be a case of Ash bringing in a new system, but rather Luke and Ash mutually committing to a defensive scheme that the defensive staff and personnel can execute at a higher speed than the past couple of seasons.
Don't believe for a minute that Withers didn't have input into the secondary system played these past two years. Luke would always be comfortable with handling the front 7, but the secondary is not his area of expertise, so I have no doubt that Withers leaving was not an unhappy issue for Meyer. Moving Coomb's responsibilities towards the Special Teams should also give you an idea of why Meyer didn't move Luke out of his position.
Bottom line - defense improves, then credit should go to the entire defensive staff, including both Coordinators.