Even without full decarbonization, my parents replaced their many-decades-old oil furnace with, IIRC, a modern gas furnace about 15 years ago. At the same time, they insulated the walls and attic of their ~1830s-era farmhouse (post-and-beam construction, lath-and-plaster interior walls) with spray-foam insulation.

Previously, they had had both an oil bill, and a fair amount of electricity that the oil furnace used to operate. After the replacement, their combined gas and electric bill was less than just their electric bill had been before.

Since then they've also added a couple of sets of solar panels, bringing the overall ongoing cost down even further. (Of course, a large portion of their grid power is already renewable—hydro power—because of where they live.)