Heat pump air feels cold because it typically is 80 degrees or so, as opposed to 110 from combustion furnace. The only retrofit that makes sense is by using either radiant heat or radiators...not forced air.

If you try a heat pump and hate it, is there any way to fix it after the fact?

How do I hate my heat pump . . . Let me count the ways.

We have gas downstairs and heat pump upstairs. It is horrible. In cold weather (which isn't even that cold) it can't keep up. It takes hours to warm up again after we've been gone. It blows air that feels cold. Aux heat turns on in weather we've been having lately which is expensive.

This is all in a new-build house from a good quality builder. If there were any way to convert it to gas, I would, but in our case, it would take major remodeling and $$$.

We have this -- a gas furnace in the main house and two heat pumps in the addition (one for the kitchen/family room, one for the master bedroom suite). It works as pp describe.

The only problem is that the air from the heat pump in the master bedroom is pushed down from the ceiling. That works great during the summer because cool air falls but it is horrible during the winter because the heat all stays up near the ceiling. Also, the blowing heat is not great for sleeping.