In what ways did Darcy change after he proposed to Elizabeth in Jane Austen's Pride and Prejudice?

In Jane Austen's Pride and Prejudice, Mr. Darcy reflects on the extent of his own changes towards the end of the book when he proposes to Elizabeth for the second time:

I have been a selfish being all my life, in practice, though not in principle. As a child I was taught what was right, but I was not taught to correct my temper. I was given good principles, but left to follow them in pride and conceit. (Ch. 58)

He further reflects that he was taught by his parents to "care for none beyond [his] own family circle," but Elizabeth changed all of that (Ch. 58).

Darcy's self-reflection refers to the fact that he has always had a compassionate and generous nature yet has also had a tendency to judge others critically. Darcy's compassionate and generous nature is reflected in the fact that he jumped at the chance to rescue Lydia by bribing Wickham to marry her, all for the sake of protecting Elizabeth's reputation.

While it was always in Darcy's nature to act for the sake of protecting Elizabeth, it was also in his nature to be in the habit of critically judging others, a flaw that was reflected in his first marriage proposal to Elizabeth. During his speech, he very openly spoke of her family's inferiority. While he was correct in his assessments, as seen in his letter to her in which he explains he made his judgements based on the impropriety displayed by her younger sisters, her mother, and even her father, he learned a valuable lesson from his experience with Elizabeth. He learned to care for others not in his immediate family circle rather than just to shun them.

The extent to which he learned to show others consideration and care is first exhibited when he treats Elizabeth's Aunt and Uncle Gardiner with such care and consideration the day he discovers them touring Pemberley. Mr. Darcy behaves so kindly and civilly to the Gardiners, some of her relations who are tradespeople and beneath his social status, that she can't help but see how much he has changed since last she spoke with him, as she reflects in her following thoughts:

Why is so altered? From what can it proceed? It cannot be for me, it cannot be for my sake that his manners are thus softened. (Ch. 43)

Hence, as we can see, Mr. Darcy has learned a valuable lesson and forced himself to be caring towards people who are not in his immediate family circle.

All in all, Darcy changed by learning to let go of his pride and conceit in order to begin to act upon the values he had been taught of being kind and considerate. Only, now, he was able to be kind and considerate while also being civil and non-judgmental.