I'm surprised this isn't more common. The only difference between a heat pump and standard air conditioner is a reversing valve. These are usually $50-$100, and just require one more wire to the thermostat. In colder weather, defrost and fancier controllers are needed, but for mild climates the reversing valve is really all that is needed.

Things like the ducts plug right in.

Other things like the blower and condenser may need to be swapped out. It also depends on how old your system is. The controller will probably most certainly need to be swapped out. Do the normal thing with contractors like that. Call 3 different dudes have them come out and give you an estimate. Tell them you want quotes for partial swap out, full swap out, and 3 different price points. Within a couple of weeks you will have most of the knowledge you need if you want to do it.

My parents when they switched out they replaced both the indoor unit and outdoor too because they were 30+ years old at that point.

Now you probably have NG? If so you can also leave that fairly in place as is. I did that with my prev house. Then have the heat pump for when it is warm outside. Then switch over to NG when it gets to a particular temp. I set it to switch over at about 30F. I could have gone as low as 15 with that unit. It worked decently for most of the time. Where I live it maybe gets in the 20s for about a week a year at most. So the heat pump worked decently.

One thing though I would say is if you have a older home especially 1990 or older start with the insulation. It is wildly cheaper to get and gets you part of the way there. Many power companies even run deals where they will help you buy it.

A "heatpump" is an AC with a reversing valve. Yes you can just get a reversing valve and have some hack cut and solder it in for you. I'd assume any normal contractor would charge $1000+ for that job though. It would take multiple hours for a tech when instead he could make multiple house calls in that same time (and possible making more money doing house calls). So that is why you won't find someone to do it. It make no sense. Right now on ebay you can get a "Goodman 4 Ton 14 SEER Heat Pump" for $1800 delivered to your doorstep.

Oh and I guess another thing a heatpump has that an AC doesn't is a defrost controller board. You'd need one of those too.

In theory but not practice. No one makes that kind of conversion kit, and there are enough "minor" differences (e.g. heat pumps need a defrost mechanism where AC's are presumed to operate with hot ambient air) to make it impractical.

I'd be very interested as well. My gas furnace is only a few years old, but my AC system is probably 10 years old. If I could replace just that with a heat pump, and leave my gas furnace in place as a backup, that would be ideal.

Not really. You replace your existing AC with a split heat pump. It's not a mix-and-match situation, but the heat pump will both heat and cool and, if you already have existing ductwork that's correctly sized, you should be fine to just drop in a heat pump. (In an extreme heating dominated climate, you could have ducts that are too small for a heat pump, but adequately sized for AC and a furnace. That's pretty uncommon though.)

You have just constructed a heat pump.

Heat pumps have the benefits of both.