In general, only items that have stats can be used in the transmogrification process. You must also be able to wear both items when using this service.
Similarly, they won’t allow you to change weapon or armor types. Sneaky death knights can’t make that breastplate look like a cloth robe, and you can’t make a one-handed axe look like a two-handed axe, or transform a sword’s appearance into that of a mace. Guns, bows, and crossbows will be the exception to this rule. You will finally be able to retain your dwarf’s racial gun bonus while appearing with all the splendor and elegance of a bow wielder (or at least the relative silence of one).