To convert over to natural gas from propane on the furnace the burner orfices will need to be changed and the gas valve will also need to be converted. This takes about an hour with the proper conversion kit.

For the water heater. Most of the time they have to be changed out to a natural gas water heater because the manufacturer wont warranty the water heater after it has been converted. If warranty isn't an issue they can be field converted with the proper conversion kit.