A productive but somewhat tense morning... I did some work on my accordions mic system, without breaking it. The existing system used a stereo jack socket, with the 'ring' connector used to switch the battery on, but that wasn't compatible with the wireless system I wanted to use. Also, as @Ventura pointed out, the circuit keeps the bass and treble mics separate, and with a studio session coming up in a couple of weeks, I wanted to bring out the bass and treble sides separately for recording.
I changed the treble volume potentiometer for one with an on/off switch built in. I also changed the volume knobs as the existing ones weren't to my taste, and were too small to hide the nut underneath. I bought a pair of 'vintage' electric guitar knobs instead. A bit of soldering later, everything is working - I can hook the accordion up to the PA in either stereo or mono depending on the cable I use, and I can use it mono with wireless.