It is very hard to perform these types of conversions in sections.
Furnaces and air handlers have specified sizes.
Performance can range from not bad to horrible with a unit that is undersized (and it will e until the system is completed).
Unico used to have hot water coils for heat that go in the air handler between the AC coil and the fan.

You are nuts to change out the best ( steam or hotwater) heating system for the lousy hot air that builders put in to minimze their costs.
Get some other opinions, and not from realters or builders. They don't have a clue.
Coincidentally,I am in the process of adding hot water baseboard heating to replace my presnt hot air system that came with my condo. A piece of junk.

Regarding difficulty in doing it in sections - The fact that the system needs to be sized correctly to get the right furnace is why I would install the furnace after all the ductwork had been installed. Remember, I plan on continuing to use the steam during the time I live there (which would likely be two to three years.)Is there any reason that wouldn't work?

Re: being crazy to change from steam - yes, I agree. Water based heat is better. But when you can't sell a house to anyone but flippers because of having what is considered an unconventional system for the area, you find yourself considering things you never would have considered before.

As to realtors not knowing - this has been an eye opening experience as to many Realtors ability to actually sell the positives of an older house. Apparently many of them do not realize that water based heat is actually better either. Young people around here have grown up with forced air heat - its what they know and are used to. Anything else scares them (and the weekly five minute maintenance of a boiler during heating system is scary to them also.)

I would consider coverting to baseboard water heat, but we currently only have a one pipe system - not a two pipe. So it sounds like it would cost a fortune to convert. If I'm spending the money anyway, I may as well go with what people know. I feel like I have to in order to sell the house for a decent price.

Also, most posts I've read that have talked about installing ductwork have used the word "gutting" to describe what is necessary. Why? Why can't a person just take out sections of the wall? Its not like ductwork is installed in every square inch of the wall. I'd much rather just have to go back and replace a two foot wide strip of drywall than a whole room's worth. Besides, many of our rooms have already been redone (before we found out about people's prejudice against steam.) I really can't see tearing them completely apart.

Thanks for the input - I do appreciate it. Good, bad and everything in between.

I have been working on a different design for heat and cool for hot climates. One fundamental change is to use solar assisted radiant heat under the floors and hot water. My best guess is that the heat can almost eliminate gas utility bills, and pay for it's self within 8 years. At that point the savings will be several thousand dollars a year for the hot water alone. We only need heat for about 3.5 months of the year. Your area would save much more with the cold you experience. Anyway, whom ever told you that steam heat was not a good selling point, is blowing smoke. What about the usage of heated floors that seems to be the norm, in cold areas? This is done with water.

That sounds creative, economical and ecological and the type of thing I would try if I wanted to stay in the house for many years, but I don't. Also, due to the fact that it is creative, we would probably have the same problem all over again when it comes times to sell. Our demographics around here do not cater to creativity.

No one told us point blank it is a bad selling point, but our realtor has "gas forced air" on the spec sheet for the house, despite our reminding her several times that it is gas/steam. Why? I think it is because she knows she won't even get lookers if its advertised as steam. Just yesterday a neighbor down the way and I were talking about our houses. His is also an older house, with Gas forced air. (Most are since they used to be coal heat so the conversion to forced air was easy due to the ducts being in place.) I told him we had steam and he curled up his nose and said "but that is so outdated." Its the reality of trying to sell the house in an overstocked market that has taught us this. True or not, people see it as outdated and hard to maintain.

So, I'm back to my original question - can I put the ductwork in for forced air gradually?

Can you convert gradually? Why not?. Once the furnace has been sized, and the duct system has been sized/designed, you can still use your existing steam system.

While installing your furnace, plenum, drop, supply and return ducts, panning, takeoffs, wall ducts, boots, registers etc, you'll just have to work around the existing steam boiler, steam piping, radiators and of course structural, plumbing and electrical obstacles.