How to let a user change their own username?
Changing a username inside a hosted site is completely up to the OpenAtrium Drupal installation.
Aegir does not usually interact with this.

I just verified that it works OK on a clean Aegir 3 install with a standard Drupal site.

So @Bisonbleu's comment seems correct: 


  That's done in permissions. In Drupal 7, go to admin/people/permissions and look under User: Change own username.