as far as I know the old control unit (ST7000) works with the new computer but it is not possile to calibrate the new course computer with the old unit.
The wrong compass reading (180 degrees) show that the system has not been calibrated, normally on the SM and SN the compass is installed looking to stern. During setup this has to be configured in the calibration.
When I changed the old course computer I also changed to control untit and left the ST7000 as a spare on board, all other functions will work with that unit.
changing only the course computer without running the calibration will result exactly in what you described...
fair winds, herbert
SN120, KALI MERA, Trinidad