Conversion kit to go to propane from natural gas is about $20 or less. Cost to go from propane to natural gas should be about the same. Going to propane involves changing the orifice and the limiter on top of the gas valve. The orifice size for propane is a lot smaller than natural gas.
I just switched a dryer back over to natural gas from propane today and noted something that may or may not be covered in any instruction sheet. The burner tube is made with an adjustable sleeve that can be rotated to change the opening for air entering into the same section as the gas coming out of the orifice. I have never seen any info on adjusting this and am asking any other readers of this forum what if anything should be done in this area when converting from Natural gas to propane or vice versa ?