How does Jimmy mature?

“A Retrieved Reformation” opens appropriately in prison. This is mainly to show that Jimmy Valentine may be a top professional safecracker and a smooth operator, but he does get caught. He is learning a couple of lessons with increasing age and experience. One is that his profession is getting tougher because he is building a record and a reputation. The other is that he is learning the shoe business and learning to like it. He also is impressed by the fact that he had to spend ten months in prison when he was only expecting to spend a few months until his connections on the outside could get him a pardon. This shows that it is getting harder to get out of prison and easier to get back in. He is losing favor with important friends on the outside. He is losing favor because he does get caught and because he is a convicted felon.
All of this is preliminary or preparatory to his sudden change of character when he falls in love at first sight with Annabel Adams. It suggests that Jimmy would have decided to reform even if he had never seen Annabel. She was not so much a "cause" as a "catalyst." O. Henry did not want his story to be simplistic, as it would have been if the protagonist only fell in love at first sight and immediately reformed. Reformation was a serious matter for O. Henry. He himself had served several years in prison for embezzlement and wrote under an assumed name in the hope of avoiding having his past exposed.