Talk with the counselors and talk with the kids. Both will give you a better idea how drastic of a change it will be.
I also agree that generally they will adapt more the younger they are. I would also try to time the transition between school changes (elementary->middle->high school) so the change isn’t as noticeable amongst classmates. It is easier to make friends if everyone around you is also trying to make friends. I think any change prior to middle school would also be less of a drastic change because at least when I was growing up kids were nice to the new kids!
As a kid, I opted to NOT change schools because I did not see the benefits outweighing everything else.
