Pretty sure Ebonlocke's transformation was a bit beyond Metamorphosis, really - Ebonlocke seems to have become a literal demon now, given he's able to be effectively banished like one. Full and permanent transformation from one state to another is always possible in the Warcraft universe, but it doesn't mean Ebonlocke occupies the same unstable niche as a Demon Hunter or what have you.
I'm not just referring to the ability of a given race to be transformed (all of them can be in extreme situations or overwhelming types of power), but more the malleability or plasticity of a given Warcraft race's biology. Elves are all over the place from an evolutionary standpoint: Night Elves, High Elves, Blood Elves, Fal'dorei, Nightborne, Wretched, Withered, Felborne, Nightfallen, and Felblood. They've demonstrated a rather extreme capacity for transformation, and so that may be a key part of how they can imbibe raw demonic essence but not turn into demons full-on, so to speak, instead holding themselves on the edge of that precipice and exist at a halfway point between Elf and Demon.