[Formendacil]

While the "apples can't turn into oranges" argument has merit, it doesn't actually work in Middle-earth because at least one orange turns into an apple (Lúthien's spirit acquires the Doom of Men), one apple turns into an orange (Tuor acquires the Doom of the Elves) and a few are Schrodinger's Fruits--Eärendil, Elwing, Elrond, Elros, Elladan, Elrohir, and Arwen.

So within the rules of Arda, fruit can change its species in certain extraordinary circumstances--and I think we can all agree that the Dagor Dagorath is an extraordinary circumstance.

Obviously, there's still a distinction between being counted as a different type of Eruhin and being counted as a Vala, but there's another angle to consider. The Valar, after all, are all Ainur. In "species" they are no different from the Ainur who remained with Eru or the lesser Ainur known as the Maiar. The question then is whether the essential fact of being a Vala their species of spirit to be Ainu or whether it is solely due to power or authority they exercise, irrespective of species--note, after all, that Melkor was once considered one of the Valar, but lost his "title," one might say, when he turned evil.