Simply put, no. Tom does not truly change in his heart. At first, Tom takes much pleasure in fulfilling the terms of his agreement - loaning money at exorbitant rates - with the Devil. In fact, the story states that he actually is quite good at it and becomes renowned for his enthusiasm and eventual fortune that he accumulates. However, as he becomes older and older the eventual damning of his soul becomes more of a fright to Tom. He thinks that reading the Bible and keeping it close will protect him from the Devil. He attends church regularly, and he even buries his old horse upside down in the event the world is turned upside down; this way he'll be ready to run from "Old Scratch." The most telling excerpt about the true heart of Tom Walker though is below:
Still, in spite of all this strenuous attention to forms, Tom had a lurking dread that the devil, after all, would have his due. That he might not be taken unawares, therefore, it is said he always carried a small bible in his coat pocket. He had also a great folio bible on his counting house desk, and would frequently be found reading it when people called on business; on such occasions he would lay his green spectacles on the book, to mark the place, while he turned round to drive some usurious bargain.
The important detail above is that despite reading the bible, he marks his place to continue making these unscrupulous and unfair deals with desperate people of the town, exploiting them and their difficult situation -- definitely not a Christian behavior. He hasn't changed at all; the church-going and bible-reading are simply an attempt to save his soul, but it isn't successful. Eventually, Tom is taken, unwillingly by the Devil and all his riches are turned to cinders.