To go to a JC5 you will have to upgrade the software of all cars in that group to HE16. The DSP should be fine and you don’t have to change any of the other CPU boards, just the software.
But before you do all that, separate the software from the board. If it is the board itself, proceed with the upgrade if you can’t get a repaired board. If it’s software there are 3 possibilities. Generic, job specific or EEPROM U22. Funny thing is none of those faults would prevent you from running on inspection. Does IMS work? If so, try a GET command just for fun. If it’s the generic (right 2), you can copy them from another car. If it’s the EEPROM (U22), you can just replace it and re-enter all parameters and do a HW scan. If it’s the job specific chips (middle 2), you will have to go to Thyssen.