Matt, the switch's contacts should certainly be kept clean, but I don't think a totally silent changeover between patterns should be expected with this type of microphone. I don't know of any "vintage" microphone that was designed specifically so that the pattern could be switched while the microphone was in the middle of picking up production sound. That just wasn't an expectation that the designers of that time sought to fulfill.
Each time you change switch positions, the DC converter disconnects one circuit branch from the rear membrane and connects a different one in its place. The switch physically breaks and makes contacts; current flows into the new branch and charges a capacitor via a resistor over a certain amount of time, and the back half of the capsule shifts its charge according to the new voltage being supplied to it via a high-value resistor.
It's straightforward, and by today's standards you might call it unsophisticated, since there's no filtering to remove switching transients or smooth the transitions. But I think that the recording engineer was expected to choose the microphone's pattern before starting a take, or between takes if an adjustment was necessary, and not during a take.
--best regards