New to heat pumps. Can I replace sub-components of an existing AC system with a heat pump for benefits of both?

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.

You are dealing with different pressures on the refrigerant lines, but honestly that shouldn't really matter all that much. You also need a bit of logic for if the condenser starts freezing over to temporarily reverse the flow (and turn off the home fan) to defrost.

But otherwise, yeah, almost identical and a little crazy they'd cost much more over a typical install.

Off the shelf heat pumps will have a defrost control board too, but you make a solid point.

The hard part is that you have to recover the refrigerant and refill, which takes HVAC/R equipment and and EPA certification to do legally.

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.

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.

You can install non ducted mini splits, which is what I did.

I left my oil furnace intact and added 4 high wall units (each bedroom and living room) with 2 outdoor condenser/compressors.

I still use my oil furnace when it gets below 40.

You can do that. It’s very common in New England to go this way because the heat pumps generally only work down to -15F and then you need to switch to furnace for heat.

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.

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.