During the ACA repair courses we were warned that changing the tuning of an accordion from 440 to 442 or vice versa is a bad idea and not good for the reeds. That is (at A4) an 8 cent difference. This will not ruin the reeds (when done with care), but it will take quite a bit of life out of the reeds. Before a reed needs to be altered by 8 cents normally corresponds to at least 3 tuning jobs (without altering base tuning) and likely even more. So when you want to change an MMM from +/-20 to +/- 10 that takes 4 or 5 tuning jobs worth of life out of the reeds, and after such a difference it is likely that reeds will go out of tune rather soon, so you're looking at at least one more tuning (of the tremolo reeds) within a year or so.This touches on a question I've had from posts here . . . Like yourself, my longtime techs regularly alter tremolo to taste for clients including touring pro's. They've never indicated that this is harmful or should not be done. ...