Although I haven't been in this situation as of yet w/ my own children, I have experienced these feelings myself. Sit your daughter down during a relaxing time of the day, and bring up the subject. Ask her how she feels about changing schools. Ask her what specific fears she has and why. Also find out if there's anything you can do on your end to help ease the transition for her. Let your daughter do most of the talking, you listen. Talking usually helps w/ situations that are uncomfortable. If she doesn't want to talk, don't push the issue. Either hold off for another time, or wait for your daughter to come to you. In any case, when you do discuss this, tell her you understand how she feels, and that her feelings are totally normal. Everyone gets a little anxious about new situations. This is normal. If you can give her an example of a personal experience in which you felt this way, maybe that will help your daughter relate better to the situation, and also she may feel she's not the only one who's ever had these feelings. Tell her that even though she'll be going to a new school w/ a new teacher and staff, she will still be around friends that she knows. Tell her she and her friends are all in the same boat and that you're sure her friends are all feeling the same way that she is right now. Every new situation takes adjusting to, but tell her she will adjust to it just like she's adjusted to other changes in her life. Again, you can use an example that she can relate to, this time in her own life. Focus on the positive. Tell your daughter that although she may feel sad leaving the comforts of her old school, w/ this change will come many new and exciting things. She will meet new friends, learn new and exciting things and have a lot more opportunities open up to her. Reassure her that she will do just fine at this new school. Also let her know that if she ever feels unsure about anything, no matter what it is, that you will always be there for her to lean on.
Someone once told me that w/ every door that closes, a new one opens. This always stuck w/ me, and I always carry that w/ me. W/ life comes changes, both good and bad. It's all in how you look at it and also choose to handle it. Good luck!