He changes drastically. He goes from being someone completely consumed by his pride and well, prejudice, to realizing these faults in himself and working to fix them. Elizabeth helped him see this in himself, and this was even explicitly stated towards the end of the novel (I could find some nice quotes if you'd like). I actually thought that both Darcy and Lizzie had an incredible amount of development; they both became humbled when it came to their "pride and prejudice" (the latter more in Lizzie's case).
Just the action of helping Lydia while accepting her humiliating position as is, in those times, is incredibly selfless and down-to-earth. Maybe in the modern day his behavior can be "still" seen as proud or, eh, conceited (which he never was, by the way - he was just never into what he considered trivial or phony social etiquette), but by the end he was definitely a changed man by all standards.