Hibernate: Change instance of base class to subclass

I would like to change a concrete superclass to one of its subclass. I've included an example below:

@Entity
@Table(name = "employees")
@Inheritance(strategy = InheritanceType.JOINED)
public class Employee {

    @Id
    @Column(name = "id")
    private String id;

    public Employee( String id ) {
        this.id = id;
    }
 ...
}

@Entity
@Table(name = "managers")
@PrimaryKeyJoinColumn(name = "id", referencedColumnName = "id")
public class Manager extends Employee {

    public Manager( String id ) {
        super(id);
    }
 ...
}


@Entity
@Table(name = "history")
public class History {
 ...
/**
 * 
 */
@ManyToOne
@JoinColumn(name = "employee_id")
private Employee employee;
 ...
}

The three classes I'm working with are Employee, Manager and History. All Managers are Employees, but not all Employees are Managers. All Employees (and Managers) have a History. Employees may be promoted to Management. When this happens an Employee's history should be retained by keeping their Employee ID the same. This will allow a Manager's History through employment to be easily found.

Implementing the promotion operation is complicated by constraints withing the database: the database will not allow removing the old Employee and creating a new Manager with the same ID without removing all of the History objects by cascading operation - which Human Resources won't allow, otherwise my job would be easy!

Is it possible to add or attach the Manager (new managers) row to an existing Employee without resorting to custom SQL operation?

I've tried the following solutions without success:

public void promote( Employee employee ) {
    /* copy over the ID of the employee to the manager.  Will not work because employee with ID already exists */
    Manager manager = new Manager(employee.getId());
    this.entityManager.persist( manager );
}

... or ...

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