This "subtle change" occurs in Chapter 16. Remember that in Chapter 15, a mob approaches Atticus at the jail to get Tom. Scout, Jem, and Dill show up and awkwardly but effectively get Walter Cunningham to convince the mob to go home.
In Chapter 16, following this event and at the start of the trial, Atticus changes a bit and this is for two main reasons. One reason is that the reality of the trial sets in. He realizes how this will affect himself, his children and the community. Another reason is that Atticus, despite these feelings of apprehension, is determined to represent Tom as best as he can and Atticus is equally determined to continue his honest and admirable ways. As a result, Scout notices Atticus has developed a subtle change in that he is more straightforward than usual and less inclined to deal with criticism.
When Aunt Alexandra criticizes Atticus for speaking openly in front of Calpurnia, Atticus reminds her that Calpurnia is a part of their family and further indicates that if there were more open discussions in the world, racial and social divisions would be less significant.
There was a faint starchiness in his voice when he said, “Anything fit to say at the table’s fit to say in front of Calpurnia. She knows what she means to this family.”
Atticus has always understood other peoples' perspectives. But this subtle change marks the moment when Atticus shifts a bit to be a bit more confrontational towards racist and elitist attitudes.