Actually you'd be surprised, a college coach will sometimes ask a player to switch to a position of need of depth or one that he feels might be more suited to that player. for instance, Sam Shields was originally a Wide Reciever until his last year of college. Nick Perry played Defensive end all through college and is now asked to convert to Linebacker. There is actually a huge difference between the levels of play, especially depending on what college conference they played in and who coached them. You have a bunch of guys who were big fish in a small pond all of a sudden being the small fish compared to guys who have been at the top for ten plus years. It's a bit overwhelming. Even when bringing a man off the bench who has been practicing, for say a year, but never actually played, there is usually an adjustment period. It is a game where you have to learn by doing, and sometimes while learning you don't do too well.
Its a difference in philosophy. 4-3 uses 4 Defensive Linemen, 2 Defensive Tackles, and 2 Defensive Ends, and 3 Linebackers. The four Defensive Linemen are asked to exclusively rush the passer. They specialize in rushing the passer with their hand in the dirt. The Linebackers are asked to drop into coverage and watch the gaps formed when the Offensive Linemen block the Defensive Linemen. In a 3-4 , One of the Linebackers are removed, the Defensive Ends are asked to stand up and play in space as Linebackers, the Defensive Tackles play Defensive End, and a larger "Nose Tackle" is inserted into the middle of the line. The Defensive Linemen are then asked to not only rush the passer, but to also watch the gaps. While the converted Defensive Ends, now called "Rush Linebackers" are asked to not only rush the passer, but to also drop into coverage. This is a very superficial explanation, and Ivo or Jack will hopefully expound, but it gets the main idea across.
