A.V.
If you really feel you must move her, PLEASE give her time in May or June to tell her friends and teachers, to get addresses and emails and phone numbers. Understand she's made friends there and if you move her, please facilitate not only a transition to the new school, but help her maintain important friendships. One of my DD's BFFs lives in a city 20-30 mins from here and I try to get her to see her friend every couple of months. I was the kid who didn't get enough of a chance to say good bye and it stank.
If she worries so much she is up at night, please find her a counselor to help her with her severe anxiety. Take her to the new school, let her meet a few people, see the 2nd grade hallway, etc. Find out when teachers are assigned and if you can have her meet her new teacher before the first day, etc. Most kids are really resilient and flexible, but I have one that is not so much so, and if I were to just not tell her all summer, she would have no time to process, ask questions, and get used to the idea. The first week would be a disaster. Don't set her up to fail. Set her up to succeed, even if she has some moments of worry. Help her address her worries.