Personally, I am always looking for an excuse to get another bike. Why not keep the mountain bike and get something truly different, like a road or cross bike? A lot of mtn bikers are scared away from road bike due to the handlebars, but drop bars just give you more alternatives for riding positions. Riding on the tops or the hoods should be little different than flat bars, and drops provide welcome relief in windy weather and long rides. Unless your roads are really bad, a road bike should be able to handle any sort of pavement and allow you ride much faster and more efficiently. You can find plenty of cross and road (touring and sport touring) bikes that will handle 32 mm tires or larger, which should be sufficient to handle almost any paved roads.