Scanivalve used to make a pneumatic multiplexer to sample the pneumatic signals in an old control room. This may still be available. I would rather replace everything with new electronic transmitters, DCS IO and digital valve positioners. Electronic transmitters have lots more precision (thus greater turndown) and are rugged and reliable. Consider replacing the pneumatic controllers. You could add a pressure transmitter on the pneumatic signal if the pneumatic controller uses a pneumatic transmitter instead of process connected. I would prefer a new electronic process transmitter wired to the DCS.
I had a local company build us a cabinet with P/I and I/P's put in the cabinet.We mounted it next to the new PLC and tubed the signals in and out from the old pnematic memonic board. It worked perfectly. We had the deltaV screens redraw to look just like the old board and the operators didn't miss a lick.
It was 1/10 the cost in 1/3 the time. The cut over was done on the fly, one loop at a time. No need for postioners as the valves and the loops were already tuned and calibrated.