Switching a player from offense to defense doesn't work very well. His formation IQ will drop to zero, and his tackling skills will usually be very low. But as mojo said, RB or WR to TE works well in some instances. I've also done LB to DL a couple of times. From what I've seen, though. You should only switch freshmen. The older they get, the more their skill ratings would drop with a switch, and they would not have as much time to regain their skills.