Until someone knowledgeable enough from the team says the "engine does not allow it", then there's no reason to believe so.
Any game with multiple appearance choices allows for characters to change them, be it with a certain GM command, or whatever. When you create a character and you choose Hairstyle 1, 2, 3 or 4, and skin tone 1, 2 or 3, for example, that data is stored in the server with your character. There is no reason to believe that that data can't be changed.
I think the simple fact that we can change between adult and kid form is already enough to show that this is also possible. Just because the game, at the moment, doesn't have a proper item/feature to do this yet, doesn't mean it can't be done. It would have to be by changing the data info related to your character by switching around a few numbers, but I do see the possibility of such a feature being added as an item later on.
