Not too hard. Primary problem is replacing the headset. It can be difficult to remove the old cups, which are pressed in to the headtube, without a special tool; then to press in the new cups, it is best to use a headset press. There aren't too many other options, or you risk damaging the cups, which will result in a headset which binds. A headset press and cup removal tool probably run about $150. Another problem would arise if your headtube is not appropriately faced, but if your current headset is OK, it is probably alright.