"specialize" a base object to become a derived object?

Is it possible to "specialize" a base object to become a derived object?

For example:

class base{...
  base(...) : ... {}//both have their own constructors
  virtual void foo(){}
}

class derived : public base{...
  void foo() override; //actual function in cpp
...}

int main(){
  base x;
  //is it possible to do something to x so
  x.foo(); // will call derived::foo()
}