SLC-5/03 and SLC-5/04 are about the only upgradeable A-B controllers that don't use ControlFlash all-software updates.
Instead, they use an EEPROM plug that attaches to the controller. You move a jumper, cycle power, watch the lights blink for a couple of minutes, and move the jumper back.
The update plug doesn't need to be permanently installed, so one plug can upgrade an unlimited number of controllers.
But it's unlikely to be available secondhand. List price is $150, from your friendly neighborhood A-B dealer. Is the application you're going to run with the new features worth that ?