I'm thinking you have a star fangled nut in the steerer and a stem cap with a bolt through it to set preload. You are wanting to change from that to an expansion plug type of cap. If the preload is set and the stem tightened, you can remove the stem cap bolt and cap, knock the star fangled nut down far enough in the steerer to make room for the expansion plug and install the expansion plug. No adjustments necessary as, once the stem is clamped down, the cap is mearly decoration.