I vaguely remember a college philosophy document a few years ago suggesting to call fumble if there was replay, but to call down if there was not. The idea being that replay can correct if it is not a fumble, but was called a fumble, but may not (at the time, could not) correct if it was a fumble but was called as down.
Calling as down seems consistent with the when in question rules - incomplete pass and not fumble, forward pass and not backward beyond NZ
The most recent TASO manual I have at hand is 2012, and it doesn't seem to mention either way regarding this.