I think you could argue that Scrooge was not likely to change. It is a little unrealistic to assume a person can go through his whole life, and then in old age become someone else. You can instead make fun of Scrooge's "shadows" and not take the whole thing seriously.
A satire should not reverse the position of Scrooge to philanthropist, but should laud him for being so conservative. Taking a sort of Johnathan Swift-like satirical approach, the writer could extol the forward thinking of Scrooge who encourages the death of the poor as "decreasing the surplus population." Then, any of the other faults of Scrooge could also be extolled. For instance, his conserving firewood in Crachit's office, his not overpaying Crachit so that he would become lax at work, etc.
(If you have not read Swift's "A Modest Proposal," you may want to look at this piece as it is a superb example of scathing satire.)
This would be a really interesting assignment to complete. My own cynically-minded self would want to reverse the plot of the story completely, making Scrooge a generous philanthropist who loves helping others with his money and is respected and cared for by the community at large, the Cratchit family and his nephew in particular. However, so annoyed with his happy, self-sacrificial behaviour, he is tormented by the ghosts into becoming a more selfish, uncaring and withdrawn individual through the visions of the past and the future that present themselves to Scrooge. He is haunted by what could have happened if he were more selfish in the past and what wealth he could have gained. Likewise he is shocked to hear the thoughts of his nephew and the Cratchit family about how foolish he is to part with his money so easily. Lastly, he is depressed to see a vision of himself dying in poverty having given all of his wealth away.
When he wakes up from his vision, he is resolved. Turning into a miserly old man overnight, he suddenly starts treating Bob Cratchit terribly, cutting his wages. He stops giving to charities and becomes famed for his misanthropic tendencies, thus exhibiting the true spirit of the age when it comes to Christmas: the selfishness and materialism that seems to dominate the season to day. The last scene cuts to Tiny Tim's death bed, as he manages to draw strength to say "God Bless us, every one," before cruelly dying from consumption. Happy Christmas!