The biggest difference we have noticed with our daughter having moved up from 10U to 12U this past year is the physical difference in a lot of the girls. Our daughter is 11, and a smaller girl...the physical difference between 11 year olds and 13 year olds can be MASSIVE, and it can make a BIG difference. We played quite a few 2nd year 12U teams, with mostly 13 year olds, and some of those girls were big...not heavy, but just physically 3-6 inches taller than my daughter, and a good 25-50 pounds heavier just due to growth spurt/puberty. My dd is a solid player, but she just physically does not have the girth to hit as hard/far or throw as fast as some of the older girls. Good luck, and hope her transition is a good one!