I doubt the team goes from a base 3-4 prevent zone defense to a complete overhaul to a 4-3. There is actually nothing wrong with the base 3-4 defense, and in the past we have not had the athletes to handle man coverage in the secondary. Arguably we do now.
What needs to happen is more diversity in the defensive play callings so that teams are scouted and the defense is adjusted accordingly (preferably pregame or even during the game). More blitzing, going dime or nickel packages, things to address particular situations (or even teams) or just to mix it up so that the opposing OC, QB and favorite WR don't play the run the WR to a gap in the zone and pitch and catch all day with the QB having all of the time in the world to do it. Our D turned the Razorbacks into a world class passing team. Our D allowed two ND players (their QB and TE) to beat us in a game we should have won. That there appeared to be no game plan for each team coming in, or adjustments made during the game, is what has the fans justifiably upset.