What ideas of changing self are being explored in Jane Austen's Pride and Prejudice?
Both Elizabeth and Darcy experience a change in self. Elizabeth realizes that she has been prideful and prejudicial while Darcy realizes that he has acted with conceit.
Elizabeth first realizes she has acted with prejudice and pride after reading Darcy's letter. In fact, she realizes that she has wrongfully taken great pride in her abilities to discern, or to judge, as we see in her lines:
How despicably have I acted ... I, who have prided myself on my discernment!--I, who have valued myself on my abilities ... and gratified my vanity, in useless or blameable distrust. (Ch. 36)
In other words, in contrast to Jane, Elizabeth believes that she has the ability to rightly judge character while her sister always sees people as much better than they are. However, it turns out that she incorrectly judged Wickham as being the best man she's known, simply because he is charming, well-mannered, very friendly, and conversational. Hence, when Wickham speaks ill of Darcy, reporting that Darcy cheated him out of his inheritance, Elizabeth felt inclined to believe him. She prejudicially accepted Wickham's account and prejudicially judged Darcy to be a despicable person merely because Darcy appears to be proud, conceited, and has a reserve. Once Elizabeth makes these realizations, she changes by becoming more accepting of Darcy and more distrusting of Wickham.
Darcy also changes himself as a result of Elizabeth's opinion of him. During his first proposal, he learns that she has always believed him to be arrogant, conceited, and selfish. She also accuses him of making a proposal in a less "gentleman-like manner" than he should have done (Ch. 34). Both accusations deeply shock and mortify him, so much so that he not only writes a letter justifying his character but also makes every endeavor to appear less arrogant, conceited, and selfish. In particular, when Elizabeth is discovered touring Pemberley with her Aunt and Uncle Gardiner, he asks to be introduced. Elizabeth notes that he seemed surprised to learn that the Gardiners were some of her working class relations that made him claim she and her family are inferior to himself, but sees that Darcy readily enters conversation with them, continues walking the grounds with them, and even invites Mr. Gardiner to fish on the estate. However, Darcy's true character is revealed when he heroically bribes Wickham into marrying Lydia, all for the sake of saving Elizabeth's reputation. During his second proposal, Darcy even confesses to feeling ashamed about how he had addressed her in his first proposal and says that her words, "had you behaved in a more gentleman-like manner," had been a source of great torment, leading him to make the necessary changes (Ch. 58).