I can't document that the bacteria are different, and what I meant was that most media are porous (more surface area for the bacteria) and it would be very difficult to remove all of the salt from these surfaces. Trying to transfer it would likely result in a slow leaching of salts for a while. With how cheap media is, it just makes sense to replace it.
However--I don't think that the bacteria are exactly the same in both systems. The entire osmotic balance issue makes it unlikely in my mind, but short of setting up a FW tank, cycling it and then converting it to SW testing all the while, I can't offer any proof. Sorry.