My Junior Martin accordion was a Bb and a friend of my changed the reeds to C. That change was no problem in this case. I think that the difference between a Bb and C are not that big. You get more troubles if you try to change the highest possible key to the lowest or reverse, because the reedblocks are to big or to small for the reeds. The brave-builders know a lot more about this and they can tell us what limitation there is.

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