Bidirectional data population design pattern

I'm looking to implement this strategy for using Symfony Form data classes in a more formal, generic way.

tl;dr: The basic idea is to capture form data into a flat, tolerant object then perform validation. If validated, populate form data into stricter, hierarchical ORM entity structure. If modifying existing data, first initialize the form data from the ORM data.

In its most simplified, unsupported, covarient form, it looks something like this:

abstract class FormDataInterface
{
  protected $entity;

  public function __construct(EntityInterface $entity)
  {
    $this->entity = $entity;
  }

  abstract public static function initialize(EntityInterface $entity): self;
  abstract public function populate(): void;
}

/**
 * Each concrete child is a two-way adaptor responsible for defining its own
 * data members and rules for populating initial form data and repopulating the
 * data back into the entity.  (In practice, this is very unique and sometimes 
 * complicated)
 */
class UserData extends FormDataInterface
{
  private $name;

  public function __construct(UserEntity $user)
  {
    parent::__construct($user);
  }

  // accessors for private members

  public static function initialize(UserEntity $user): self
  {
    $userData = new self($user);
    $userData->setName($user->getName());
    return $userData;
  }

  public function populate(): void
  {
    $this->entity->setName($this->name);
  }
}

The problem is the specialization of the EntityInterface to a specific concrete entity. The rules of the system indicate that each concrete form data class will be paired with a specific entity. Also, I want to be able to define the initialize() and populate() methods somehow in the contract.

I've read through this issue. Both the Strategy Pattern and Bridge Pattern seem viable, but I'm not clear on how either address the covariance problem.