Changing object type in runtime using reflection

No, it is not possible. Reflection that you mentioned allows discovering given class at runtime (i.e. finding method, field etc by name) and accessing methods and fields of objects at runtime without compiling the client code against specific class. For example you can invoke method foo() of any class.

Changing type of object actually does not make sense for me at all. Object contains is an instance of speicific class that have both data and methods. I can somehow imagine way to change the memory allocated for object at runtime using sun.misc.Unsafe, however I even cannot imagine how can you change the implementation of methods done in specific class associated with the object.

And the question that still remains here: why? Could you probably explain your task and ask for solution proposal?

EDIT

Following the new information posted by OP as a comment to my answer I'd like to add the following.

As far as I understand the situation is the following.

There is a third party library that implements class A and AFactory. OP uses code like the following:

A a = AFactory.create();

However he does not need A. He needs B extends A that implements additional functionality.

Possible solution is the following.

Create class B extends A:

public class B extends A {
    private final A a;
    public B(A a) {this.a = a;}
    // delegate all methods of A, i.e.:
    @Override
    public boolean isA() {return a.isA()}

    // add your functionality, e.g.
    public boolean isC() {/* your code here*/}
}

Now use this class as following:

A a = AFactory.create();
B b = new B(a); 

Now your can use all functionality of A via B and the additional functionality as well.