When in doubt, please order the thermostatically controlled models. In that case never install a non-modulating heater such as the EX95. When ordering make sure to only order a heater that has a modulating valve and also it's a good idea to install an additional scald protection valve before your showerhead. Sorry, but we don't. Even if we could, we probably wouldn't, as in most cases it is not a good idea to convert one type of gas for another.
Burners can be different, orifices, etc.
Not only is the BTU rating different for these two gases, but also the heat of the flames and a number of other factors. A conventional flue is what most people are familiar with. It typically consists of a double-wall flue pipe going from the top of the heater through the roof, venting outside. A direct-vent unit on the other hand, both vents and draws its combustion air through a specially designed pipe from the outside via an adjacent wall. Your products specify the 'temp.