It appears that even an admin cannot edit their own username from the frontend.
Community Builder allowed the user to both change their username on the frontend and have a username space.
In addition to this the admin could choose whether or not a user should be able to change their username.
So what I'm proposing is to have two options in the backend. One to choose whether frontend users should be able to change their username and another to choose whether to allow spaces or not. These options could either go under the custom field area or the Joomla Username Application.