In Harper Lee's story, To Kill a Mockingbird, Atticus doesn't change as much as the children do. He is a man of intellect with a keen sense of right and wrong. He is their moral compass.
Atticus's own actions in arguing the Robinson case demonstrate this kind of courage, and his behavior throughout embodies values of dignity, integrity, determination, and tolerance.
Atticus remains this way throughout the kids' dealings with the Radleys (when they are trying to get Boo to come out), with Mrs. Dubose and the flowers Jem destroys, and the court cast of Tom Robinson; he is even consistent at the end when he thinks Jem has killed Bob Ewell. Not even to protect Jem does he want to act a lie, or he's afraid his children will no longer respect him, and won't see the world in the same way.
'Heck,' Atticus's back was turned. 'If this thing's hushed up it'll be a simple denial to Jem of the way I've tried to raise him. Sometimes I think I'm a total failure as a parent, but I'm all they've got. Before Jem looks at anyone else he looks at me, and I've tried to live so I can look squarely back at him...if I connived at something like this, frankly I couldn't meet his eye, and the day I can't do that I'll know I've lost him. I don't want to lose him and Scout, because they're all I've got.'
Atticus thinks he is getting older quickly, and over those couple of years, it may seem that way to him: of course the kids think he is ancient.
As the children grow, they perceive Atticus differently: as when they learn he had to take Tom's case (though the man Atticus is would never have said no) and that he is a good shot with a gun. They begin to see him differently as they mature.
However, if anything changes in Atticus it would be how he is different after Tom Robinson's case. He says that every lawyer has one case that really affects him, and he figures this was the one for him.
And as for the night he almost loses the children to Bob Ewell's attack, it takes something out of him. I think he was not enough aware of the evil in men's hearts, as he never thought Ewell would come after his children instead of him. The split second when they were almost lost to him forever had to be a riveting moment for Atticus the father.
All in all, Atticus is the one constant in the book, just as a parent should be a constant in his/her child's life.