Most any kid will have some kind of issue in the transition so don't panic. The ball size is 1 of those issues but 12u plays with a 12-inch ball. No need to champion for anything else so accept it and get ready. The good news; she won't need to change anymore. lol. To be truthful, most girls seem to struggle more with the extra 5 ft. of distance than the ball size. The release point changes and if a 10u kid is borderline with her level of skill this will definately have an impact. Bigger ball, move her back, and plenty of bucket time for mom and dad will get her ready.