The headset pieces combined total up to what is called the stack height. When the new fork is installed the steerer tube is cut to a length that allows for the stack height + the length of the head tube through which the steerer passes. Normally, that steerer length is cut to be a millimeter or two shorter to allow for that top nut to clear the top of the steerer. If yours does not you should be able to add a spacer to the stack and use the nut that goes with the headset.