I think switching mid-year can work, and DS was fine, but he did feel bad about leaving friends behind. It's not nearly as hard switching at the beginning of the school year, when everyone is kind of mixed up anyway and looking for new friends. DD switched classes due to grade acceleration mid-year as well and I think by the time she moved a lot of the girls already had their "best friend" and kind of ignored her. She ended up making friends with another new girl that came shortly after her (and everyone ignored the other new girl as well). It was a kind of odd class where the kids didn't seem particularly sociable. But she was fine. The kids were much friendlier when DS switched for whatever reason (both did mid-year switches in first grade). The upper grades are probably harder for girls than first grade.
