Replacing Giant Defy 4 2013 alloy steerer fork with Giant carbon steerer fork
As other have stated you should be able to use the old headset providing it is in good condition. If you are swapping forks due to an accident then replace the headset even if it feels smooth. Your alloy steerer likely used a "star fangled nut " to apply the headset preload. With a carbon steerer you need a expandable plug that is made for use with carbon steerers.