The theme of "The Christmas carol" is, change does not come in an instant nor does it not take effort, Change takes time to settle in and perhaps it is unnoticeable to the person experiencing change until they look back and see the person they used to be.
"Oh, no. kind spirit! Say he will be spared." if these shadows remain unaltered by the future none other of my race," returned the ghost," will find him here. What then? If he be like to die, he better do it, and decrease the surplus population."(648)
"I am here to warn you that you have yet a chance and hope of escaping my fate. A chance and hope of my procuring, Ebenezer."(629)
Marley is directly telling Scrooge he still has a chance, all he needs to do is make a change for the better. This is the first part of scrooges change therefore the theme. Scrooge notices the need for change and this starts to already slowly change him.
"I will honor Christmas in my heart, and try to keep it all year. I will live in the past, the present, and the future. The spirits of all three shall strive within me, I will not shut out the lessons that they teach."(663)
"Nothing. There was a boy singing a Christmas carol at my door last night. I should like to have given him something. That's all."(635)
"Really, for a man who had been out of practice for so many years, it was a splendid laugh, a most illustrious laugh."(666)
Scrooge is gradually improving, he progressed from being remarkably rude and mean to the Christmas carolers to wanting to give one of them something. This is a slow and minor improvement and Scrooge doesn't seem to notice but it is there, He is improving.
Scrooge really begins improving in this quote, He went from not regarding whether people perished, and actually prefuring for those so-called" worthless unproductive people" to die. To aspiring for Tiny Tim to continue to live and grow strong, he hoped that he would become healthier and survive. He also begins to comprehend himself from a different perspective which enables him to observe the person he was and compare that to the way he currently is.
This is the moment when all the ghosts laborious effort ultimately was meriting, Scrooge is a different man. He is reformed, redeemed. It took time but ultimately, he observed himself from a different perspective who he was and that he needs to improve. That ultimately drives him to mature into the kind man he is after the story concludes.
Scrooge is fully improved, It took time and effort but it was done, essentially the reason he changed was that he saw who he used to be, and how he improved, then he saw the consequences of continuing to be a bad man.