How does Jimmy's decision to use his burglary tools to save Agatha show a change in his character in "A Retrieved Reformation"?

Jimmy's skillful use of his burglary tools clearly demonstrates his willingness to return to prison rather than have a child suffocate. His change of character is from partial self-interest to complete unselfishness.

After Jimmy Valentine changes his name to Ralph Spencer, his transformation from thief and safe cracker to good citizen begins. This change is made because he has fallen in love with pretty Annabel Adams, the daughter of the bank president in Elmore, Arkansas, where he originally intended to rob the bank. It is also because of his love for Annabel that he decides to give away his tools to another man. This change to good citizen is partially because of his own self-interest in wishing to marry Annabel Adams.

Fortunately for nine-year-old Agatha—but unfortunately for him—Ralph Spencer happens to still have the safe-cracking tools in a supposed suitcase for his trip to Little Rock when he steps into the Elmore Bank on the day that the little girl has trapped herself inside. When Annabel pleads with her fiancé, "Can't you do something, Ralph—try, won't you?" he looks at her with "a queer, soft smile on his lips and in his keen eyes" because he knows that he must act nobly for his beloved Annabel, or lose her devotion. He also loves her and does not want her to be distraught.

So, in his most unselfish act of love, Ralph returns to being Jimmy Valentine and breaks into the vault, saving Agatha. Fully realizing that Annabel and her family now know that he has previously engaged in criminal activity, Jimmy walks out of the bank, thinking he has lost his beloved. But Ben Price rewards him for his truly loving act, aware that Jimmy is, indeed, now reformed. When Jimmy speaks to him, resigned to being arrested, Price pretends that he does not know Jimmy.