Keep your Symfony form's password value around if an empty value is submitted. Optionally supply a "Clear Password" field.

KeepPasswordListener.php

<?php
/**
 * Copyright (c) 2016 PMG <https://www.pmg.com>
 * 
 * License: MIT
 */

use Symfony\Component\PropertyAccess\PropertyAccess;
use Symfony\Component\PropertyAccess\PropertyAccessorInterface;
use Symfony\Component\Form\FormEvents;
use Symfony\Component\Form\FormEvent;
use Symfony\Component\EventDispatcher\EventSubscriberInterface;

/**
 * Use inside your form types.
 * 
 *    public function buildForm(FormBuilderInterface $build, array $options)
 *    {
 *      $build->add('password', PasswordType::class);
 *      $build->add('remove_password', CheckboxType::class, [
 *        'mapped' => false,
 *      ]);
 *      $build->addEventSubscriber(new KeepPasswordListener('password', 'remove_password'));
 *    }
 */
final class KeepPasswordListener implements EventSubscriberInterface
{
    /**
     * The name of a password field whose value should be kept if an 
     * empty value is submitted.
     *
     * @var string
     */
    private $passwordField;

    /**
     * The boolean (checkbox) field that tells the listener to clear
     * the password.
     *
     * @var string
     */
    private $clearField;

    /**
     * @var PropertyAccessorInterface
     */
    private $accessor;

    public function __construct($passwordField, $clearField=null, PropertyAccessorInterface $accessor=null)
    {
        $this->passwordField = $passwordField;
        $this->clearField = $clearField;
        $this->accessor = $accessor ?: PropertyAccess::createPropertyAccessor();
    }

    /**
     * {@inheritdoc}
     */
    public static function getSubscribedEvents()
    {
        return [
            FormEvents::PRE_SUBMIT => 'maybeKeepPassword',
        ];
    }

    public function maybeKeepPassword(FormEvent $event)
    {
        $pwField = $event->getForm()->get($this->passwordField);
        $formData = $event->getForm()->getData();
        $submitData = $event->getData();

        $erasePassword = false;
        if ($this->clearField && isset($submitData[$this->clearField])) {
            $erasePassword = self::asBool($submitData[$this->clearField]);
        }

        // if we're erasing the password we can get out of here straight away
        if ($erasePassword) {
            $submitData[$this->passwordField] = $pwField->getConfig()->getEmptyData();
            $event->setData($submitData);
            return;
        }

        if (empty($submitData[$this->passwordField])) {
            $submitData[$this->passwordField] = $this->accessor->getValue(
                $event->getForm()->getData(),
                $pwField->getPropertyPath()
            );
            $event->setData($submitData);
        }
    }

    private static function asBool($value)
    {
        return filter_var($value, FILTER_VALIDATE_BOOLEAN);
    }
}