Switch from standard to config_installer with an existing db on Drupal 8, via https://www.drupal.org/project/drupal/issues/2677722

custom_module.install

<?php
 /**
 * @file
 * Install file for the Custom Module module.
 */
 
 /**
 * Ensure site is using 'config_installer' profile.
 */
function custom_module_update_8001(&$sandbox) {
  \Drupal::configFactory()->getEditable('core.extension')
    ->set('profile', 'config_installer')
    ->save();
  drupal_flush_all_caches();
  
  \Drupal::service('module_installer')->install(['config_installer']);
  \Drupal::service('module_installer')->uninstall(['standard']);
  
  $sc = \Drupal::keyValue('system.schema');
  if ($sc->get('standard')) {
    $sc->delete('standard');
  }
  drupal_flush_all_caches();
}