POSITION CHANGES in the NFL tend to be incremental. A nickel cornerback who is more quick than fast moves to safety. A guard with nifty hands and a good grasp of how defenses line up moves to center. An athletic quarterback with unexceptional passing skills tries the other end of the equation, as a wide receiver.
Bigger changes, such as the one the Eagles' Taylor Hart is trying to make, from defense to offense, are rarer. It's the NFL. You're competing against guys who started learning what you are trying to learn back when they were toddlers. Unless you already have a framework, a context - the nickel corner already has mastered some of the leverage and positioning issues he'll face at safety - you're apt to look silly, regardless of how athletic you are.