Originally Posted by Scrodzilla
I was under every impression the OP is just switching his fork and using his existing headset.
this is the impression i got too. is that right OP?
that said, what you need to do is (provided you are not crossing ISO with JIS as stated) unscrew the lock nut, then the top cap (removing any spacers), pull the fork out. then you must remove the crown race from the existing fork and put it on the new fork. sometimes this is easy, sometimes this is very difficult without the proper tools. it's made to sit tight and a crown race tool is usually necessary to put it on the new fork, even if you can get away with removing it from the old one with a fat screwdriver and a hammer. the important thing is to NOT DAMAGE the race where it comes in contact with the bearings, and it should sit flush against the crown.
provided you can do that, just add some grease and reinstall. of course you have to know how to properly adjust a threaded headset. do you know how to do that? if you don't you will ruin your headset by either having it too tight or too loose.