Changing of the pipes may be an issue of "codes" in your particular town. The contractor should know what has to be done. If you have copper lines, they probably need replaced with black pipe. Also, all appliances that currently fire with Propane will need the orifice's changed out to Natural gas. Natural gas fires at a lower pressure than propane so this has to be done. Conversion kit's are availiable for most major appliances.

Thank you Hollywood. We already have black pipe and my real problem is that one contractor has said that we would need to replace the pipe and another has said that we would not need to. Both of them are reputable contractors and I just cannot figure what is going on. We were aware of the need to change the orifices, but we have also been told that the water heaters that are fitted for LP will need to be changed entirely. Does this sound normal? Thanks again.

The existing black pipe in the house is OK according to the National Fuel Gas code. The water tank has to be changed out. The manufaturer does not provide conversion kits for the tank. A new tank should only be about $275-375 installed.......Look at it this way,...you'll get a warrenty with it. I usually go with a 10 year tank because you cant beat the price.