I am not aware of any major changes to the main PCB except with the swap from AC to DC motor. Would have only been minor changes after that I think!
The only exception is when the 4004, then the 6000 was released the former had remote datalink (I use the word 'data' rather casually here!) and the latter a CD4 / RIAA board, both PCB's being placed under the keypad with no major changes to the main board. Shame there was never a 6004 - I guess would have needed a new lead and DIN with the extra wires!