Change Symfony Form-field in Form-event

FormType.php

 $builder->addEventListener(FormEvents::PRE_SET_DATA, function (FormEvent $event) {
  $data = $event->getData();
  $form = $event->getForm();

  $config = $form->get('groups')->getConfig(); // get the field config
  $name = $config->getName();                  // get the name string
  $type = get_class($config->getType()->getInnerType());  // get the type class
  $options = $config->getOptions();            // get the options array

  $options['label'] = "Login Name";   // e.g. change the label

  $form->add($name, $type, $options); // replace the field
});