There may have been some changes in the laws since your heater was built which preclude changing the gas type. I do not know this to be fact, but just something you may learn as you search for someone to make the change. In spite of the direction on the tank, this is not something a homeowner should take on. You can make a mistake plumbing water and just get wet. Make a mistake with gas and you get a very loud BOOM. One thing to consider. Your heater has only a few years at best left before it will have to be replaced. Maybe maybe you should look at replacing the heater now.