Altering the username that is displayed for a user

Thanks to the hook_user_format_name_alter() hook altering the username that is displayed for a user is really easy. Let's say that you have the First and Last name fields on the user entity. You can do something like this to alter the username:

use Drupal\Core\Session\AccountInterface;
use Drupal\user\Entity\User;

/**
 * Implements hook_user_format_name_alter().
 */
function MY_MODULE_user_format_name_alter(&$name, AccountInterface $account) {
  $full_name = [];

  $account = User::load($account->id());
  $first_name = $account->get('field_first_name')->value;
  $last_name = $account->get('field_last_name')->value;

  if (!empty($first_name)) {
    $full_name[] = $first_name;
  }

  if (!empty($last_name)) {
    $full_name[] = $last_name;
  }

  if (!empty($full_name)) {
    $name = implode(' ', $full_name);
  }
}

If you now try to get the username by using this $account->getDisplayName() you will see the new username that consists of the first and last name.