I think there are two ways to accomplish this.
1: Connect via serial cable into a receiver and restore it to factory defaults. Then you can go into the network settings via the serial menu and detect the transmitter. Finally, you will then be able to connect serially to the transmitter using a direct CAT5/6 cable via the receiver and change the transmitter IP through the serial menu.
2: (Easiest way) Connect serially into the HMX manager, change the IP of the Eth0 port to a 192.168.13.xxx address and save the changes (does NOT require rebooting). Then you should be able to discover the transmitter normally through the web browser menu. Change the IP of the transmitter through the browser, and then delete the device from the manager. Change the IP of the HMX manager back to your 192.168.51.xxx address and re-discover the transmitter with the new address you gave it.
Pain in the tail, but we have done both with success.
-Ben