$20k sounds a lot to me. What peak power did you need (BTU/h or kW)? Have you shopped around for offers?
Changing from gas to air-to-water heat pump involves electrical work (inside and out), core drilling through structural brick walls, some landscaping work, much more plumbing work (both water and refrigeration), and more and more expensive equipment (way more piping, a buffer tank, a domestic water storage tank, and the indoor and outdoor units), plus the labor to install, inspect, and maintain all that.
Heating load on design day was calculated 78KBTU/hr. (Old boiler was ~60% efficient, 200KBTU/hr input and cycled easily on design days.) I think the true figure is likely just a bit less, but the exact figure turns out to be irrelevant for a gas combi boiler as it’s already sized large enough for the higher domestic water heating load. For a heat pump, sizing to load and using storage tanks is critical.
I really wanted to have the switch make sense. It wasn’t even close, even with several thousand in additional incentives.
When climate becomes enough of a problem that governments start taxing fossil fuel instead of subsidizing it, the story may be different. But for now I would have made the same choice.
The old boiler really was a disaster from an efficiency standpoint. 1950s General Motors (not a typo) oil burner, converted to gas with a single-stage [200K or 0 BTU/hr] burner, drawing combustion air from the basement, and sending 160°F-190°F water to the building and taking up an enormous footprint in the basement. Now has a 95% Bosch combi, drawing combustion air from the outside, using outdoor reset to send 118-136°F water to the building, and hangs on the wall taking up about as much space as two milk crates stack atop each other.
Our grid is ~40% natural gas anyway, so when taxes hit natural gas, they're going to hit heat pump users as well.