EF4仅编码TPH更改对象类型

我只是想用EF4代码来解决一些问题。如果我使用TPH,并且我想把一个获救的人改成讲师,或者反过来,我该如何完成这个任务呢。我的POCO课程:

    public class Person
{
    public int PersonId { get; set; }

    public string FirstName { get; set; }

    public string LastName { get; set; }
}
public class Instructor : Person
{
    public DateTime? HireDate { get; set; }
}

public class Student : Person
{
    public DateTime? EnrollmentDate { get; set; }
}

public class Admin : Person
{
    public DateTime? AdminDate { get; set; }
}

public class PersonConfiguration : EntityConfiguration<Person>
{
    public PersonConfiguration()
    {
        this.HasKey(u => u.PersonId).Property(u => u.PersonId).IsIdentity();
        MapHierarchy()
            .Case<Person>(p => new
            {
                p.PersonId,
                p.FirstName,
                p.LastName,                    
                PersonCategory = 0
            })
            .Case<Instructor>(i => new
            {

                i.HireDate,
                PersonCategory = 1
            })
            .Case<Student>(s => new
            {
                s.EnrollmentDate,
                PersonCategory = 2
            })
            .Case<Admin>(a => new
            {

                a.AdminDate,
                PersonCategory = 3
            }).ToTable("Person");

    }
}

假设我有一个人:

var person1 = new Person { FirstName = "Bayram", LastName = "Celik" };
context.People.Add(person1);
context.SaveChanges();

以后我想让这个人成为管理员。我该如何做到这一点。

var person = context.People.FirstOrDefault();
Admin test = person as Admin; // wont work

下面将更改HireDate列,但我的discriminator字段PersonCategory仍然是0。所以就EF而言,它仍然不是管理员类型

Admin admin = new Admin();
admin.PersonId = person.PersonId;
admin.AdminDate = DateTime.Now;

context.ObjectContext.Detach(person);
context.People.Attach(admin);
var customerEntry = context.ObjectContext.ObjectStateManager.GetObjectStateEntry(admin);
customerEntry.SetModified();
customerEntry.SetModifiedProperty("AdminDate");