I'm afraid there's no simple change to qa-page-user.php that would allow the admin to change the username of a user via that user's page.
But the good news is that Q2A 1.4 will include an option to prevent users changing their usernames, once they have at least one post.
Apart from that, you're safe to directly manipulate the username in the qa_users table in the database - nothing needs to be recalculated.