It is possible for a natural gas water tank to be converted to propane. The water heater must be labeled as convertible, and only a licensed gas technician should convert the appliance.
The label on the appliance should always be referred to. Tanks that are able to be converted from one gas to another are clearly labeled as such and often come with a conversion kit.
Converting an appliance requires the replacement of gas orifices. Converting the tank to propane requires compensation for the differing pressures, as natural gas is a lower pressure gas than propane. The gas orifices are made to work with a specific gas and gas pressure. Natural gas orifices are larger than propane orifices because of the pressure of the gas.
Along with replacing the orifices when converting the water tank, appliance regulators must also be replaced. The differences between propane and natural gas regulators include inlet and delivery pressures. The regulators provide the correct amount of pressure depending on the type of fuel.
Even when conversion kits are available, a licensed technician must perform the conversion. In the end, it may be cheaper to purchase a new tank than trying to convert one that is designed for natural gas use.