A.E.
Hey C.,
I was a Navy brat so we had to change schools alot! I would just try to focus on accentuating the positives. If there is someone she might not like at her old school, she wont have to deal with them at the new school. Generally, though, honesty is the best way to handle the situation. You need to verbally acknowledge that this will be a hard adjustment, but then stress that change is good. (I am sure you have some good personal examples!) Let her express her concerns and then let her handle it... She'll do more amazing than you may expect!
Best of luck! A.