How do I programmatically change a system message?
I was seeking a similar solution and found this great post describing using a hook_preprocess_HOOK(), in this case, hook_preprocess_status_messages() - https://stackoverflow.com/questions/38788080/remove-default-message-in-drupal-8

For my purpose to override a message, I use a simple strpos in the logic:

/**
 * Implements hook_preprocess_HOOK().
 *
 * @param $variables
 */
function mymodule_preprocess_status_messages(&amp;$variables) {
  if(isset($variables['message_list']['status'])){
    $status_messages = $variables['message_list']['status'];
    foreach($status_messages as $delta =&gt; $message) {
      if (strpos((string) $message, 'The message that I am expecting') !== FALSE) {
        $variables['message_list']['status'][$delta] = "Some different message text";
      }
    }
  }
}