I'm told that they can convert to salt water by becoming dormant for 24-48 hours and then they can re-convert back to freshwater the same way.

A few years ago when we had all those floods that wiped out Mozambique, I did an ariel game count along the Crocodile River (RSA) about a week after the floods subsided and the entire area was devoid of flatdogs.....normally it's stiff with them. The theory was that a large percentage had been washed out to sea and converted to salt water..... I wouldn't comment on that as I don't know enough about it, but by the next time we overflew the same area (a year later) they all seemed to be back to their pre flood numbers.