What details suggest that Jimmy has really changed?

O. Henry suggests that Jimmy undergoes a sudden transformation when he first sees Annabel Adams.

A young lady crossed the street, passed him at the corner and entered a door over which was the sign “The Elmore Bank.” Jimmy Valentine looked into her eyes, forgot what he was, and became another man. 

Is this possible? There must have been other factors leading up to Jimmy's transformation. No doubt he was beginning to realize that his success as a safecracker was making him so notorious that it would lead to his undoing. The ten months he recently served in prison for "that Springfield job" gave him plenty of time to reflect. This story is mainly about the transforming power of love, and the moral is "Crime does not pay," or "Honesty is the best policy." His change begins with Annabel and is a conspicuous fact throughout the story. He cannot remain a criminal and hope to win such an angelic girl. She is a symbol of honesty and respectability. She loves him and only sees what is good in him. He wants to be deserving of her.

Another detail suggesting that Jimmy has really changed is his decision to get rid of his suitcase full of custom-designed safecracking tools. He is burning his bridges behind him. He decides to give the tools to an old pal and writes in his letter to him:

Say, Billy, I've quit the old business—a year ago. I've got a nice store. I'm making an honest living, and I'm going to marry the finest girl on earth two weeks from now. It's the only life, Billy—the straight one. I wouldn't touch a dollar of another man's money now for a million. 

A third sign that Jimmy has really changed is his sacrifice of everything--his business, his reputation as a respectable citizen, his fiancee, and his freedom--in order to save a little girl who has accidentally gotten locked inside a supposedly burglar-proof bank vault. It is ironic that Jimmy uses his expertise and specialized tools for a good purpose rather than a criminal one. This noble sacrifice is proof of his love for Annabel and proof of his reformation. Although he thinks he has sacrificed everything by breaking into the bank vault in front of a number of astonished witnesses, it puts the seal on his reformation. Ben Price, a tough, case-hardened lawman, sees what Jimmy does and allows him to retain his new identity as Ralph Spencer, an honest businessman and pillar of the community.

“Guess you're mistaken, Mr. Spencer,” he said. “Don't believe I recognize you. Your buggy's waiting for you, ain't it?”