This is a good question, but perhaps a better way of looking at things is to say that Atticus did not change as much as Scout. There is a strong argument to be made that Atticus was consistent. In fact, he is the most constant in the whole book. Therefore, when we speak of change, perhaps it is better to speak of Scout's change of perception.
At first, Scout did not think much of her father. She thought that he was old and weak, unlike the other fathers.
Atticus was feeble: he was nearly fifty. When Jem and I asked him why he was so old, he said he got started late, which we felt reflected upon his abilities and manliness. He was much older than the parents of our school contemporaries, and there was nothing Jem or I could say about him when our classmates said, “My father—”
However, as Scout grows older, she realizes that her father is wise and courageous, even in his raising of children. Later she recalls an incident that happened to her and she realizes that Atticus was parenting her in subtle ways. Scout confesses:
I scurried to my room and went to bed. Uncle Jack was a prince of a fellow not to let me down. But I never figured out how Atticus knew I was listening, and it was not until many years later that I realized he wanted me to hear every word he said.
In light of this, it is best to say that Scout's perception of her father changed.