Casting vtkSmartPointer to an Inherited Class

How do I cast a vtkSmartPointer<T> to an inherited class while maintaining reference counting?

Minimal illustration:

#include <iostream>
#include <vtkSmartPointer.h>
class A: public vtkObjectBase {
public :
    A(){}
    static A * New(){return new A();}
    int var1 = 8;
};

class B: public A {
public :
    B(){}
    static B * New() {return new B();}
    int var2 = 12;
};

int main (int argc, char ** argv) {
    vtkSmartPointer<B> b = vtkSmartPointer<B>::New();
    vtkSmartPointer<A> a = b;  // this is fine
    std::cout << "var1 = " << a->var1 << std::endl;
    // this is not fine and I cannot find a vtk equivalent
    // to boost's dynamic_pointer_cast for similar functionality
//    vtkSmartPointer<B> c = a; // how do I do this?     
}

I'm assuming there must be a macro somewhere similar to boost's dynamic_pointer_cast<T> but I can't find it. If there isn't, and someone can suggest a method to accomplish this, I would be very grateful.