Since kerosene is a liquid and propane is burned as a gas, converting from kerosene to propane can be prohibitively expensive. Almost all parts related to combustion need to be switched, which means purchasing a number of parts and using them in a furnace not designed for propane heating.
Another problem with converting a kerosene heater to a propane heater is the precision of newer furnaces. Systems are designed holistically, and each component is chosen to maximize the system's efficiency. While converting between gas fuels can be done in a manner that conserves this efficiency, converting from a liquid fuel to a gas fuel is likely to squander some of the furnace's design features.