I don't think I'm quoting directly, but instead paraphrasing something from that great yogi, the Buddha:
If you want to know a person's past lives, look at his present condition. If you want to know a person's future lives, look at his present action.
This seems to indicate that we can change our karma right now.
On a related note, Joe Biden said the other day, "Don't tell me what you value. Show me your budget and I'll tell you what you value."