I wouldn't bother going through the bother of Park tools instructions on freehub overhall. They are cheap enough just to replace. The problem is to take them off to replace, you need to remove the axial. This is why you end up doing a hub overhall. The best way to do it is just to remove the locknut and cone on one side of the axial. This way you can just repack the hub with grease and replace the removed locknut and cone and not worry about redishing the wheel or anything.
A cone wrench is 8.99 at the shop I work at, and is porbably the only extra tool you would have to buy, besides the 15mm allen. Tools are always a good investment, or you could just have your LBS do it.
-ray