But if you use that gas furnace to heat radiators in a fluid-born (usually water) heating system, you only need to install an electric heater next to your furna

But if you use that gas furnace to heat radiators in a fluid-born (usually water) heating system, you only need to install an electric heater next to your furnace and re-route the pipes to/from the furnace to use that heater in stead. Not all that humongous an operation.