Karma is not synonymous with destiny. It is very important to understand that without any doubts or confusion. Destiny is merely the end result of Karma. Karma includes both freewill and destiny. Out of freewill we decide to do something, and as a result of that decision we create a destiny for ourselves. The entire process, the freewill decision and the destined result, are included in the concept of Karma.
What we can change about karma is the freewill part of it. We cannot change what destiny we have already created. But we can change the destiny we will create. Therefore we can change what we do with the destinies we find ourselves in. By so doing, we actually change the future course of our destiny.
We simply cannot change our destiny at all in any way shape or form. We cannot modify or influence or do anything to it. We have 0% control over that. But we have 100% control over how we react to our destiny, how we use it, what we do with it.
A simple example. It is my destiny that a knife shall be placed on my desk. It is up to me what I do with that knife. Will I slice bread or commit a crime? It is already beyond my control that there is a knife on my desk. That is a result of things I did in the past. But it is entirely in my control what I do with the knife in the future.
The true purpose of karma is to bring about our enlightenment, by putting us into inescapable situations that are require us to effect an effort for self-improvement, or which reward us for ways we have previously improved the use of our freewill.
“Improvement” means becoming more selfless.
The Universe Herself is the witness and enforcer of karma. It is she who witnesses the actions we do of our own freewill. And therefore it is she who prepares the resultant reactions which are appropriate to bring about our self-improvement and eventual enlightenment. The planets and stars themselves are part of Her, part of the Universe. They move with the movements of the universe and thus indicate to us what destiny the universe will cause to unfold in our lifetime, as a result of our previous actions.