I couldn't find anything in the owners manual (click on "documentation" in the Defender listing at the link you provided) that even mentions flipping the discharge port, so I think it would be a good idea to talk to tech support at Jabsco before taking anything apart. Paul Compagna is their toilets etc guru...his direct line is 978-282-5246.
I think you can rotate it 180 degrees. If my memory is correct, the 4 bolts that hold the end on are two short ones that hold the macerator end to the impeller body, and two long ones that go all the way through to the motor. I don't believe there's any sort of key or registration to prevent rotating. I currently have a spare motor and housing separated on the boat. If no one else confirms it for you by then, I'll check in the morning when I get down there.