The piece you want to change will simply change look with the purchase. A new piece of gear is not created. Its just like reforging, except instead of changing stats the look changes. And there is an undo button, just like reforging. Its a single piece of gear, it looks normal or transmogrified into something else.
Also, those with Corrupted Ashbringer will be the single most epic looking players. A full tier 3 set with Corrupted Ashbringer will be the single most badass looking ppl. I wonder how many of those old school naxx raiders kept all nine pieces of their tier....if they even got all nine.