Drupal 8 Custom form mode

module.php

<?php

/**
 * Implements hook_entity_form_display_alter().
 */
function MY_MODULE_entity_form_display_alter(&$form_display, array $context) {
  if ($context['entity_type'] == 'node' && $context['bundle'] == 'MY_BUNDLE') {
    $current_user = \Drupal::currentUser();
    if ($current_user->hasPermission('CUSTOM PERMISSION')) {
      $storage = \Drupal::entityManager()->getStorage('entity_form_display');
      $new_form_display = $storage->load('node.MY_BUNDLE.FORM_MODE_ID');
      if ($new_form_display) {
        $form_display = $new_form_display;
      }
    }
  }
}