There is nothing like a predetermined destiny or calling. Your decisions and desires are influenced by things you are exposed to. That particular thing you so much desire now or feel like is your destiny or calling is a product of the things you have been exposed to right from birth. The totality of your environment. For instance a child that caught his or her dreams of being an engineer, through a particular television series or the kind of things and people he is exposed to in his immediate environment, might not desire engineering profession if exposed to a different environment with different elements and individuals.
The actual thing that propels people towards a particular career is not destiny or calling but interest and desires. When one picks a genuine interest on something it turns into a burning desire which consumes that person personality, and makes him see himself only through the eyes of his desire. He then builds this believe that he can only be good at his current area of interest and nothing more. Thus, he sees it as his calling or destiny.
So to say, interest and desires are the underlying factors of what we call destiny or calling. There is 99% chance that you might not be what you are today if you were to be exposed to a different environment and ideologies right from birth. So what you are today is not predetermined outcome, it is a product of your interest and desires which guides the choices you make in life. If your interest were to be something else, it will affect your life choices and in the long run, who you are today.
Summarily, when your interest changes, your choice changes and in the same manner, your personality changes. You have no predetermined destiny or calling, you become that which you want to be, consciously or unconsciously.
Your life is being mold in accordance with the choices you make in life. If you want a change in you, then you will have to re-align your thinking in the right manner and your choices as well.