Thats a difficult one.
Its difficult to know what your destiny is until it happens.
But I do believe that if somehow a master wizard or some such tells you what your destiny is, you can make an effort to try and change it.
Although maybe, if it is your destiny and what is meant to happen, the forces of the universe will conspire against you and make what is meant to happen happen.
I am not sure I belive in destiny being something that will definately happen. It goes against the idea of free will.
You may know your destiny, but it is your choice whether you accept it.