Really? I did not know that. But then, I keep my items sorted by type, so Poke Ball is at the top of the list of balls.Actually mine is set to Beast Ball. I organized them alphabetically and its at the top. Whichever is closest to the top is pretty assigned when pressing Y.
So in that way... you can change the pre-selected ball for the Y button. All you have to do is put the desired ball at the top of your bag's way-too-crowded general items pocket. Now I wonder if placing the balls in the Free Space pocket affects where they show up in the ball menu in battle...