Everything points to the fact that fh5 to fh conversion represent re-saving of drawings made in Adobe FreeHand 5 to (.fh5) to older .fh files that were used in previous version of the program.
We do not know about any other 3rd party program that can handle FreeHand graphics, so your only choice for this conversion would be to find a copy of the program (version 5 of higher) and hope that it can save the drawings between versions.