There’s a few different angles on this question. That’s why there tends to be debate about it. Most of us human’s lack the patience and broad-mindedness to be able to hear and understand more than one angle at a time. I will try to present an answer which addresses most of the angles.
Karma pertains to the actions you have performed in the past. In that sense, no you can’t change this aspect of Karma. It’s impossible to change something you have already done.
The actions you freely chose to take in the past create the destiny you deal with in the present (and future). In this scope of the term it is possible to change your karma. To understand this you first need to understand that Karma is the universe’s way of helping us to become enlightened, gradually. She, mother nature – the universe, observes actions we take of our own freewill, and she keeps us responsible for those actions by rewarding or punishing us with “destiny” that will either encourage us to continue in the better, more selfless things we are doing, or discourage us from the selfish things we are doing.
So karma is just the way that a mother interacts with her children. We are the children, and the universe is our mother. If you are a mom or dad with kids, it will be pretty easy for you to understand: When kids do something wrong and don’t even realize they did something wong, you have to get angry, chastize, etc. Worse when they do something wrong and refuse to admit it was wrong, or lie about it – then you really need to be disciplining. But in situations where they do something wrong and really feel sorry about it, you don’t even need to reprimand much. Because you know they will make efforts to correct themselves. This is exactly how our mother, the universe, cares for us regarding karma. When she sees us trying to be proactive about improving ourselves and moving towards becoming enlightened, she doesn’t really need to make our karmic reactions so strong and punishing, because we already have displayed to her that we understand right from wrong and are working on it ourselves. When we don’t even care about being selfish and wrong in our life, or when we make excuses, or don’t even care to understand right from wrong, that’s when she needs to be more disciplinary with the exact nature of the karma she dispenses.
– Vic DiCara