I researched and shopped it pretty hard (the latter to limited effect). Lots of companies do boiler swaps in ~3 hours with one plumber or one plumber and an apprentice/laborer. Only one company would even quote the air-to-water (which has its own dangers now and for 20 years of operation).
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.