Peter, I have difficulty going from one size accordion to another. My guess is that it's because I started playing the accordion rather late in life. When you can't see your fingers or what they're doing, you have to rely on muscle memory. I'm guessing it takes longer to train muscle memory for a task that's learned later in life than it does for something that's learned when we are younger. I've met accordionists who don't have any trouble changing sizes, but most of them have been playing since they were young.

Skipjack may be an exception to this. I think he started playing accordion in his thirties, and he seems to be able to switch sizes without too much difficulty, but he's also a very talented musician, something I can't really say about myself.

That's my theory, anyway.