One might look simply at the changes in Scrooge's attitude from the beginning of the first visitation to the end of the last. When the ghost of Christmas past suggests that he come and take a walk with him, Scrooge is annoyed at the prospect of having to get out of his warm bed. He feels that his "welfare" would be best addressed by just getting a night of rest. There is nothing about his life that feels particularly amiss to him.
By the time he reaches the grave laid out for him by the final spirit, Scrooge is desperate to have the opportunity to change. He begs and pleads with the spirit to have the chance to go back and try to right some of the wrongs that he has seen in his long night of visions. By this point Scrooge is desperate to change and goes so far as to clutch at the outstretched hand of the terrifying spirit.
It is also useful to look at the way that all those he interacted with once he came to his senses were shocked by the change in him. At first they assume he is making some cruel joke; they cannot believe he has honestly changed, but his jovial and earnest manner soon convinces them.