In addition to what Al posted:
http://forums.usfirst.org/showthread.php?t=14310
The thing to be careful of is the model number. To be legal it must be a end with -9015. We have gotten different motors in the past, my team hasn't been around very long, but we have a pair of -9012 motors laying around.
So bottom line is yes you may use them, provided they are the correct model and they are being used in place of ones from this years kit. 2 FPs total allowed on the robot.