The party<em>could</em>switch their aspects like that, but they would have to bind enough elementals to cover the 10 motes per character it costs to switch elements, then deal with the side effects, which can be any effect which falls under the purview of the emotion, illusion, and sickness keywords, and tends to be extremely unpredictable, so there's nothing wrong with saying that trying that with a terrestrial is fatal.  According to Dreams of the First Age, the medicine charm Transcendant Gaian Harmony enables a dragon blood to get a second elemental aspect at Essence six, and another one at essence 7, 8, and 9.  At essence 10, the charm can be purchased a sixth time, although nobody knows what that would do.  (Personally, I wouldn't do it for fear that one wraps around to vitriol and gains that elemental aspect.)<br/><br/>I only have a problem with the Elemental Dragons being used as an explanation, since they're said to be even less caring about the world than the jade pleasure dome crew are.  Though I also don't mind playing in a variant setting where the elemental dragons personally exalt the terrestrials and personally give them blessings and curses.