Then there wouldn't much point to RP as High Elf if there is nothing about about that character that resembles a High Elf. People might RP as they like, but I highly doubt people who are interested in High Elves would settle for that. If they could turn into the way Alleria turned into the void, a more controlled transformation then there could be a possibility.
I mean, if they recruit now, it would be logical that they do it in a controlled matter, aka Alleria way. So far the Void Elves we play with are the ones that came with Magister Umbric. Giving a going out of form thing like Alleria would go with the idea that they are recruiting. If the story reflect this, then I can't see why not. It wouldn't make everyone happy, and nothing ever does, but that would actually show what are you are trying to get through, that Alliance High Elves also go the Void Elven way. So far nothing in the game reflect this directly, but changing this customization would change a lot.