Maybe I can help clear this up. First, we all know that propane has more heat potential than natural gas at the same pressure. The reason for this is that propane has a specific gravity of 1.562 at 70 degrees. Natural gas has a specific gravity of .667 at 70 degrees. When propane is introduced into natural gas lines, it is first mixed with air to bring its specific gravity down. So, when you open up the lines to reconnect to the natural gas meters, some air will enter the lines bringing the specific gravity of the remaining propane closer to that of natural gas. There will be no ill effects to the appliances.