BD, not an installer, just built a house with the spray foam from the start (even spray foamed some interior walls for sound deadening). My house is all electric so I didn't have to worry about gas appliances and the possible buildup of fumes. I was actually not aware that you could put gas appliances in a foam house when I first started planning (about 9 years ago), so I always planned on all electric. I would be leery of not switching out the units. My understanding now is that the gas units should be enclosed and completely sealed off with outside venting to prevent the gases from getting back into the house, since the whole purpose of the foam is an airtight seal.
I can tell you that the house is pretty airtight. We have to crack a nearby window to get a fire started, otherwise it'll barely smolder. Once it's going, it creates a draft with a separate fresh air inlet specifically for the fireplace only.