The shuttle is synchronized so you don't have to be at complete stop when you move the lever to change direction, but try to picture the stresses on the drive system if the tractor is still moving rolling one direction and you let out the clutch to go opposite. All the wieght/energy of the tractor is going one way, now the drive system is immediately trying to force it opposite direction. If no other stress damage ever occurs, you will wear the clutch out sooner.
You may be able to push in the clutch and shift while rolling, but come to a stop (or very close to it) before letting the clutch out.