Nope, you don't neck up, what you do is cut the shoulder of the .223 off right below the shoulder and then lubricate and put in a sizing die, anneal the new neck/shoulder which is now sized for a 30 caliber bullet. The .300 uses the same magazine as the .223, so the only thing you really have to change on the gun is the barrel, instead of building just a new upper, I built a whole new gun. Right now I am waiting for someone to get new stripped uppers in stock and I will be building a new .223 so my wife and I can shoot at the same time.
Oh by the way, as a new AR owner, you are going to want to read my thread in Dockside about what happened after a couple of bumpfire dumps on Friday, not fun, very painful, but a very good reminder of how hot these guns actually run!