I would never in my life tell anyone to go from hotwater heat to hot air. The best jobs in the world are hotwater, the cheapest is scorched air. The pros and cons are endless. Allergies, hot air is bad. Comfort, hot air is bad, it has no residual your heating air while the burner is on and that's it, when you heat water it stays warm and has some mass to it. Add an a/c system, maybe get an upgrade on the boiler. If you go to hot air you will regret it in the end for many many reasons. Ask any one that has it, or better yet any one that has switched.