On the hardware side, I think it's only minor differences:
Inside mine, I have IC301 populated (LT1081 , an rs232 driver) near the main CPU (MC68332), and a ribbon cable from PL301 (near IC301) that goes to the DB9 panel connector.
I don't think it's an "upgrade kit" that you can purchase. The missing hardware can probably be added easily but it may also need different firmware (not sure, I know mine has some code for the GPIB option even if it's not installed). I would be curious to compare ROM dumps of other units with different options but I'm not aware of anyone else having uploaded theirs.