Yes, it is possible for a person to change completely, but normally such a transition takes a greater period of time than, say, a 24-hour period. To change an attitude or behavior can take weeks, months, or even years, depending upon the person. If a person has experienced a supernatural or religious event, then they may start out with the best of intentions, but to completely alter one's self will still take time for adjustment. Human nature is such that an immediate change is almost always temporary if that change is not trained or developed.
Not only do I not believe it is possible for people to completely change, I don't believe that Scrooge completely changed. I don't believe that the Scrooge in the beginning of "A Christmas Carol," was the true nature of the man. When reading the story of the young boy with the Ghost of Christmas Past we learn that Scrooge was a sad lonely boy, sent away by his father. This is where the anger and rage began. We learn Belle, Scrooge's old girlfriend broke her relationship off with him because she felt that he had changed for the worse."
"Scrooge finds himself at the school that he attended as a boy, watching all of the other children leaving for Christmas. He is shocked to see a young Scrooge, a lonely but imaginative boy that daydreams about characters out of "Ali Baba" and Robinson Crusoe. Suddenly it is the same scene a few years later, when Scrooge's little sister, Fan, excitedly tells him that their father said he can come home this year."
Scrooge became as a man, hard and angry, but his soul was not so. As soon as he was shown the past, present and future he decides to return to the type of person he wanted to be all along. It was life's experiences that created the "adult Scrooge," and it was a desire to become better that changed him. He was not totally bad to begin with so it was not a complete change from his original nature.
Yes, but it would take a tremendous self-awareness that most people do not possess. Even with that awareness, implementing and committing to change is difficult, especially to the degree presented in the character of Scrooge. Possible? Yes. Probable? No.
I do not think that it is likely that people can change completely as Scrooge does in this novel but I do think that it is possible. In the story, Scrooge goes through a complete transformation because of the past, present, and future events that are presented to him. He actually sees what he has done, how he became what he has become, and what will really happen to him if he does not change his ways. As people outside of the realm of a novel, we are not able to see back into the past nor can we see a true vision of the future so most people would not be able to go through this change. However, it is possible that some people, given their past and how they see themselves in the present can speculate what their futures will bring. If this future is not what the person whats for himself or herself, then yes, he or she might be able to make that drastic and complete change as Scrooge did in the novel.
I think a person can only make a complete paradigm shift in their life through being self reflective and having something life-changing happen in their life. You hear people talking about the 7 points in their life that changed their way of thinking about the world. Usually this is a major event, followed by a new self awareness in the world.
Speaking from a personal experience, after watching my grandmother and uncle die in the hospital after a house fire, I felt a whole different respect for life and the time that I have here and now.