The compromise in that situation is that the new improved Blood Elves keep their animations, the old models get their stances change like the Nightborne had their original Night Elf stances changed to what they are now. These changed stances, (I would how for a more humble looking stance) perhaps using the even older Night Elf version of the model animations, since they still use the modified Night Elf voice files for many of them, would be Alliance High Elves. The hand-me downs of the Elf races to give to the poor Alliance who can't come up with a reasonable Allied Race to save its soul.
You would have two different feeling groups of elves that look similar, but are not identical. They might be the same or similar in color and perhaps hair colors, but they would hold themselves differently and would actually be different models. The Blood Elves, or Sun Elves maybe after they slowly mutate a bit from their new source of power, will keep their arrogant (to my eye) stance....were they look bored with whom ever is in front of them, and sigh every once in a while, like they'd which you'd stop breathing their air. Meanwhile the High Elves (or former High Elves, should they deserve a rename) will not hold themselves up so highly. They might seem tired, or weary. Their stance not as proud looking as others. They know what is happening to them as a people and are accepting it. They might survive, but they will struggle a lot and have to live with the humans, perhaps for many elven generations (which means they will see a lot of their human friends die....even if it is only due to old age....because elves still live a long time without the Well of Eternity or the Sunwell/Nightwell.
The Void Elves will probably keep their present model and stance, since they would be between what will become a Blood Elf/Sun Elf, and the old High Elves. Older model, Blood Elf stances, plus Void stuff.
See, three different groups. Not that hard to imagine.