The field upgrade was sold by Moog directly at the time they also sold the new digital Voyager board to upgrade from one to many banks of sounds. Moog did not sold these field upgrades for many years.
I bought one aftertouch modification in 2009. You received a board (see attached picture) that you had to exchange with the board in the left hand section near the wheels. It it improve the aftertouch from being rather on/off characteristic to an average working one. Rudi Linhard, the one who did the Voyager Software and was close to Bob Moog, did another update with a pcb of way more parts. I installed it myself (actually swapping my left hand section and sending back the old left hand section back to him) and got now a superb aftertouch response. I am able to get any value from 0-127 in a very detailed manner. This is by fat the best aftertouch response in my hole collection of synths.