At the beginning of the play, one could imagine that Don Pedro might dominate the play's action. He seems to enjoy being in charge, probably as a carryover from his role as prince and military leader. He not only supports Claudio's bid for Hero's hand, but he offers to help arrange it. When the marriage is arranged — after what he sees as a minor misunderstanding — he's ready to get Benedick's life rearranged with Beatrice.
When Don Pedro is deceived by his brother's presentation of the "window scene" between Borachio and a woman he sees as Hero, he is probably shaken by how readily he had been taken in by an innocent-seeming Hero, even helping Claudio to win her. He does not see through his brother's latest act of treachery, even when it's further denied at the wedding. He tries to make light of the consequences when Hero is reported as dead but again must recognize his own gullibility when he hears of his brother's treachery from the mouth of the guilty Borachio. Never again does Don Pedro appear as sure of himself as he was at the beginning of the play. Even at the final wedding scene, amidst the general merriment, Benedick notices that "Prince, thou art sad. Get thee a wife, get thee a wife."
Has Don Pedro changed? Perhaps as much as any character has, as a result of recognizing his own vulnerability and his ultimate inability to keep control of events around him.