In William Congreve's The Way of the World the aspect of Mirabell's behavior that suggests that he is "reformed" is that he seems to be finally able to have feelings, true feelings, for only one woman. His feelings for Millamant are new to him. He was once a rake who would fool around, sleep around, be treacherous to his servants, two-faced to his peers and certainly the biggest of the ladies' men.
However, his choice to love Millamant results in that he now has a broken heart. No rake ever bothers being heart-broken. They just go with the flow and live for the moment. Yet, the fact that Mirabell even thinks of Millamant as a future wife and the fact that she is obviously the focus of his affection is one of the biggest factors that could lead us to believe that he has, indeed, changed.