There are many types of ammonia and nitrite comsuming bacteria in fresh and salt water. The majority only live in one type, but there are nitrifying bacteria that live in both and these are usually the ones they culture for cycling products that can be used in either. Keep in mined that the strongest and fastest growing nitrifying bacteria in either will be ones more specialized to there environment. If you switch from one to the other you would lose part of your bio-filter, how much would depend on how big a part of your filter is specific to that water type. In a tank that has been up for a long time the vast majority of the bio filter will be the bacteria specialized to that water type.