I honestly believe that Kallis played conservatively because he was asked to be the bedrock of the team for much of his career. He was the prized wicket, the person who would rebuild to a decent score.
I think since Amla burst onto the scene ( last 4 years), Kallis has started to play with much more freedom. He realises or knows that Amla is the new bedrock and He ( Kallis) can play his normal game.
T20's have also improved Kallis as a player IMO. The turning point was when he was left out of the T20 World Cup because he was percieved as being to slow. He works on his game continously and is constantly reinventing himself to stay relevant.
One has to remember that as All rounder, he is by far and away the greatest of his generation. How many other all rounders are still doing their stuff at 37? Mad Respect for kallis.