Point of clarification:
If you're replacing only the headset, then yes, you can probably get away with using the existing star nut that is in the fork. IMHO, even if the new headset has a lower stackheight and you are going to cut down a little of the existing steerer tube (why you'd do this and not just add a spacer I don't know, maybe if you were an extreme weight weenie), you could even get away with driving the existing star nut further down.
If you're replacing the fork, then DeadSailor's advice applies.
Either way, relax and have fun doing the upgrades!