Yep, when I was in the bay area, we had a gas furnace that was built in week 53 of the year before the 78% efficiency standard took effect. Our plan was to put in a heatpump to replace it when it stopped working and get A/C to manage (at least) the heatwaves, but it kept going, so you know. That house was built in 1907 and had an unfinished basement for the original footprint and a generous crawl space for the addition --- wiring would have been pretty simple, but it would still be a lot of investment to replace something that was working ... of course a heat pump system would be way more than replacing like for like.