Hierarchy of value objects

1.cs

public class Person : Entity
{
    public virtual string Name { get; set; }
    public virtual Document Document { get; set; }
}

public class Document : Entity
{
}

public class IdentityCard : Document
{
    public virtual DateTime IssueDate { get; set; }
}

public class Passport : Document
{
    public virtual string SerialNumber { get; set; }
}

2.cs

var person = new Person { Name = "Name" };
person.Document = new IdentityCard(DateTime.Now);
Flush(); // both the person and the document are created

person.Document = new Passport("Serial Number");
Flush(); // the new document is assigned but the old one remains in the database

3.cs

public class DocumentContainer
{
    private DocumentType _type;
    private DateTime? _issueDate;
    private string _serialNumber;

    public Document Document
    {
        get
        {
            switch (_type)
            {
                case DocumentType.IdentityCard:
                    return new IdentityCard(_issueDate.Value);

                case DocumentType.Passport:
                    return new Passport(_serialNumber);

                default:
                    throw new ArgumentOutOfRangeException();
            }
        }
        set
        {
            switch (value)
            {
                case IdentityCard identityCard:
                    _issueDate = identityCard.IssueDate;
                    _serialNumber = null;
                    _type = DocumentType.IdentityCard;
                    break;

                case Passport passport:
                    _issueDate = null;
                    _serialNumber = passport.SerialNumber;
                    _type = DocumentType.Passport;
                    break;
            }
        }
    }
}

public class Person : Entity
{
    public virtual string Name { get; set; }

    private readonly DocumentContainer _document;
    public virtual Document Document
    {
        get => _document.Document;
        set => _document.Document = value;
    }
}