What do you think C. S. Lewis wants the reader to learn from the change that Digory undergoes in The Magician's Nephew?

All of the books in the Narnia series have strong moral
lessons, and in this one the main character, Digory (the professors' nephew) grows from
a self-centered boy into a young man with principles. In the beginning of the novel,
Digory is a miserable, whiny kid that is feeling sorry for himself because he is in
London with his uncle and sickly mother. He likes to do risky things and does not think
about the consequences. As a result, he and his friend, Polly, get into a lot of
trouble.

However, as the novel progresses, Digory changes.
He starts to think more about his actions. He matures. He learns to put people ahead of
his own selfish wants. When his uncle Andrew comes under the spell of Jadis, the evil
witch, Digory is the one who decides to bring her into Narnia. In doing this, he
releases sin into that world (similar to the Adam and Eve story in the Bible). Luckily,
though, Narnia is ruled by Aslan, whom Jadis cannot defeat. Aslan gives Digory a chance
to redeem himself and sends him to retrieve a magic apple. Digory resists eating the
apple, even though he wants to eat it very much and take one for his mother as well. In
the past, he would not have hesitated to eat it. He now realizes that his mother would
not approve of him stealing something, even if it will heal
her.

In making the choice not to eat the apple, Digory
illustrates that he has learned one of life's most important lessons: sometimes doing
the right thing is hard and requires extreme sacrifice. Since Digory makes the right
decision, Aslan rewards him and gives him the apple. Digory's mother is cured, and
Digory plants the apple which grows into a tree, the wood of which he uses to build the
famous WARDROBE that begins the other stories.

Lewis'
message, I believe, reflects the Biblical truth that just because something is difficult
does not mean we should not do it, if it is the right thing to
do.