Discount Above Ground Pool Heaters

I am going to get a pool heater for my above ground pool. Although I have an above ground pool, I'd like to know if I can install an inground pool heater. My pool is 15 feet by 30 feet and I'd like to install a 200,000 BTU/Hr heater but I can only find inground pool heaters in that size. Is there a difference in these two heater types that would preclude me using an inground heater on my above ground pool?

I have found a used 250,000 BTU Hayward pool heater that I'd like to use to heat our pool. It's a natural gas fired unit, but I'd like to use propane instead. I understand that you can convert natural gas pool heaters to propane, but I don't know what is involved. Do I need an additional part? Any help is appreciated.