For most home applications all your need is to replace the jets in the appliance - $.25 in parts. Stoves and dryers commonly come with both sets, and I've seen furnaces that can be converted. I've never seen a water heater than can be converted, though it is in theory possible.
The hard part is an entire neighborhood must be converted at once. You have all summer to convert the furnace, but things used all summer like stoves, or water heaters have to be done for the entire neighborhood in one day. This means a lot of logistics to get the right parts and labor in place to do the work. Though if people are willing to accept propane takes outside their house you can delay a bit, but it needs to be done before everyone needs heat.
You forgot heat pumps, that is what will be everyone switching to. It's more efficient to burn propane centrally in a power plant and use the electrical grid to power heat pumps than distribution/burning of propane locally.
While heat pumps are a great idea, where I live they don't work on the coldest days so you need some other backup. Burning propane on those cold days is more efficient than a central power plant.
The hard part is an entire neighborhood must be converted at once. You have all summer to convert the furnace, but things used all summer like stoves, or water heaters have to be done for the entire neighborhood in one day. This means a lot of logistics to get the right parts and labor in place to do the work. Though if people are willing to accept propane takes outside their house you can delay a bit, but it needs to be done before everyone needs heat.