Change symfony security password - Not FOS

I'm using the standard symfony security for my admin password:

security:
    providers:
        in_memory:
            memory:
                users:
                    admin:
                        password: $2y$12$.QD1HlEVmeupiIIM3d601urQxd.WsgJyfZ0nJjVAyEn2qQOGpjIHi
                        roles: 'ROLE_ADMIN'
    encoders:
        Symfony\Component\Security\Core\User\User:
            algorithm: bcrypt
            cost: 12

Now I would like to implement a method to change the password. With some tweaks I managed to get the answer from this question to work for the password checking aspect.

This is my password class: I was forced to use the public type for now because I kept getting the Could not determine access type for property error. Side question, why would this be happening? I had the setters and getters uncommented

<?php
namespace AppBundle\Security;

use Symfony\Component\Security\Core\Validator\Constraints as SecurityAssert;
use Symfony\Component\Validator\Constraints as Assert;

class ChangePassword
{
    /**
     * @SecurityAssert\UserPassword(
     *     message = "Niepoprawne Haslo"
     * )
     */
    public $oldPassword;

    /**
     * @Assert\Length(
     *     min = 6,
     *     minMessage = "Hasło powinno mieć min. 6 znaków"
     * )
     */
    public $newPassword;

    // /**
    //  * Get oldPassword
    //  *
    //  * @return string
    //  */
    // public function getOldPassword()
    // {
    //  return $this->oldPassword;
    // }

    // *
    //  * Get newPassword
    //  *
    //  * @return string

    // public function getNewPassword()
    // {
    //  return $this->newPassword;
    // }

    // public function setNewPassword($newPassword)
    // {
    //  $this->newPassword = $newPassword;

    //  return $this;
    // }

}

My type:

<?php
namespace AppBundle\Form;

use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\FormBuilderInterface;
use Symfony\Component\OptionsResolver\OptionsResolverInterface;
use Symfony\Component\Form\Extension\Core\Type\PasswordType;
use Symfony\Component\Form\Extension\Core\Type\RepeatedType;
use Symfony\Component\Form\Extension\Core\Type\SubmitType;
use AppBundle\Security\ChangePassword;


class ChangePasswordType extends AbstractType
{
    public function buildForm(FormBuilderInterface $builder, array $options)
    {

        $builder->add('oldPassword', PasswordType::class, array(
            'label' => 'Stare Hasło'))
        ->add('newPassword', RepeatedType::class, array(
            'type' => PasswordType::class,
            'invalid_message' => 'Hasła nie pasują do siebie',
            'required' => true,
            'first_options'  => array('label' => 'Nowe Hasło'),
            'second_options' => array('label' => 'Wprowadź ponownie'),
        ))
        ->add('save', SubmitType::class, array(
            'label' => 'Zmień Hasło',
            'attr' => array(
            'class' => 'btn btn-danger'
        )));
    }

    public function setDefaultOptions(OptionsResolverInterface $resolver)
    {
        $resolver->setDefaults(array(
            'data_class' => 'AppBundle\Security\ChangePassword',
        ));
    }

    public function getName()
    {
        return 'change_passwd';
    }
}

And finally the controller:

public function adminChangePasswdAction(Request $request)
    {
        $changePasswordModel = new ChangePassword();
        $form = $this->createForm(ChangePasswordType::class, $changePasswordModel);

        $form->handleRequest($request);

        if ($form->isSubmitted() && $form->isValid()) {

            $data = $form->getData();

            $user = $this->getUser();

            $plainPassword = $form->getData()->newPassword;
            $encoder = $this->container->get('security.password_encoder');
            $encoded = $encoder->encodePassword($user, $plainPassword);

            $user->setPassword($encoded);

          //return $this->redirect($this->generateUrl('admin-panel'));
        }

        return $this->render('admin/changepasswd.html.twig', array(
          'form' => $form->createView(),
        ));      
    }

But as you can easily guess the $user->setPassword($encoded); portion is not working because the user class does not have a setPassword method.

What would be the proper way to set the password? Is it even possible?