All good questions from James, but a lot of the times the system has to be upgraded because the IC board controlling the hydraulics is no longer made.
As for the solutions, is the system actually controlling something with a PID loop for example, or is it just driving the proportional solenoid?
If it is a direct control of the output solenoid, one option is to control a motor that moves the pot for you. I never would have thought about this as a solution, but I have a few machines with it. It is ****, as expected.
Depending on how the input of the card where the pot is connected, there could be a chance of being able to connect the IO card directly. I would say though that this is either unlikely or you wouldn't have the documentation to do it.
Lastly, if you know the valves you are controlling and what signal they take (they may not take signal as such and just react on current, bear in mind the number of solenoids), then you can get an hydraulic amplifier or driver (I have a soft spot for Wandfluh) and connect that to your analog output on the PLC.
Either way, do bear in mind what the system does and more importantly the current standards for said system because once you touch it, it has to meet current standards.