What would change if the story focused on males rather than females, and how is the change in Mr. Darcy's character convincing?
The change would be pretty radical. Men are interested in all sorts of things, including war, exploration, business, sports, travel, science, technology, invention, and drinking and gambling. The women in Jane Austen's books seem to be interested almost exclusively in getting married or getting some other woman married. They are also interested in things related to getting married and having children, including dresses, balls, and all kinds of social activities that will give females opportunities to meet eligible males. Jane Austen is quite popular with contemporary female readers, but I don't think many man like her novels. Here is what Mark Twain had to say about Pride and Prejudice:
Every time I read Pride and Prejudice I want to dig her [Jane Austen] up and hit her over the skull with her own shin-bone.
And here is what a more recent writer, Andy Rooney, had to say:
I have never read anything Austen wrote. I just never got at reading Pride and Prejudice or Sense and Sensibility. They seemed to be the Bobbsey Twins for grown-ups.
I confess that I have the same problem with Jane Austen. It seems to me that she understands women very well but does not understand men at all. The men in her novels seem to exist solely as potential "catches" or as "providers," like poor Mr. Bennet with a houseful of dependent women, after being caught. Her male characters always seem to be hovering around women looking for wives, as if that is all men ever thought about. The men are always "calling" on women at their homes and attending social functions in order to look at women, dance with them, flirt with them, and so on. I get the strong impression that Jane Austen's women are shallow and materialistic. They want men to love them, but they do not love back; they want men to provide a house, servants, security, money, clothes, more clothes, social status, and everything else they desire.
Mr. Darcy is a marvelous "catch" because he is tall, dark, handsome, and virile, and because he has an income of five thousand pounds a year (off other people's labor), if I remember correctly. His change of character is not at all believable to me, or to most men, I should think. Elizabeth Bennet, like Beauty in "Beauty and the Beast," brings Darcy to his knees by the simple expedient of not fawning on him and thereby hurting his pride. I think he literally gets down on his knees and begs her to marry him, even though she has no dowry or social status and may not even be good looking. She uses reverse psychology on the poor guy and he can't stand it! This makes him look like a sap, but a lot of Jane Austen's male characters seem like saps.