Board Thread:Fourth Databook/@comment-4366149-20141122004623/@comment-4271790-20141203015900

The thing is, shaping a nature does not advance it. For example, when Sasuke shapes Chidori into Chidori Sharp Spear, the Sharp Spear doesn't advance to a new lightning nature. That's part of the reason why Amaterasu and Kagutsuchi have the same nature.