code: def send_username_changed_email(self, user):
          
  
          # Verify config settings
          if not self.user_manager.USER_ENABLE_EMAIL: return
          if not self.user_manager.USER_SEND_USERNAME_CHANGED_EMAIL: return
  
          # Notification emails are sent to the user's primary email address
          user_or_user_email_object = self.user_manager.db_manager.get_primary_user_email_object(user)
          email = user_or_user_email_object.email
  
          # Render email from templates and send it via the configured EmailAdapter
          self._render_and_send_email(
              email,
              user,
              self.user_manager.USER_USERNAME_CHANGED_EMAIL_TEMPLATE,
          )
docs: Send the 'username has changed' notification email.