Atticus never changed throughout the whole novel. However, through Scout's narration, we find that Atticus was a true gentleman. He never told them that he was "appointed" to the Tom Robinson case. He could have, but he did not. It did not matter to him either way. He believed in what he was doing and was not about to give excuses for why he was doing it (defending Tom). That was in part two of the link--summary.
Another topic that is important to point out is that we never knew until he shot that dog that he was the best shot in town. He kept it to himself, where others may have bragged. (the end of part one of summary in attached link)
Our admiration for the man only grew from the beginning of the novel until the end.