Tom Walker does not change as he ages; he merely grows fearful of the hereafter because he has sold his soul to the Devil.
Washington Irving writes that as Tom grows older he begins to feel anxious about the next world:
He thought with regret on the bargain he had made with his black friend, and set his wits to work to cheat him out of the conditions.
So, Walker becomes "a violent churchgoer," praying vociferously so that heaven can hear him. He becomes sanctimonious in his practice of religion and attendance at church and his acting as a "stern supervisor" and "censurer" of his fellow-churchgoers:
...he seemed to think every sin entered up to their account became a credit on his side.
Nevertheless, Tom still worries that the Devil will collect his due. So, he tries to not be taken unaware and carries a small Bible in the pocket of his coat. At his desk, he keeps a larger Bible and makes the pretense of reading it whenever someone comes into his office.
It is clear that Tom Walker dreads the day of reckoning with Old Scratch. As a young man he was greedy and eager to be wealthy, not worrying about the conditions of his pact with the Black Man. However, as he ages, Tom begins to worry about the hereafter, so he becomes superstitious, thinking that if he prays and attends church and keeps a Bible out, he can thwart the Devil.
Certainly, the satire of Irving is apparent in the denouement of his story in which all of Tom's religious efforts fail. For, one day Tom leaves his little Bible at the bottom of his coat pocket and the large Bible is buried beneath a pile of mortgage papers to be foreclosed, so the "black man whisked him like a child...and away he galloped."