You need to have 2 different looking pieces of the same equipment slot.
Say 2 different looking chest pieces or 2 different looking blades.
Now you can choose to change the look of one of the pieces to the other one.
You will just be changing the look, the stats will still be the same as before.
Also, you don't lose the other piece when you take its look.
Additional info:
This costs a certain amount of steel (around 150) to do, so you may want to add that to the information as well. ~
doowylloH