Before you decide whether to let her transfer, have her revisit the priorities that led her to make her original decision. If her priorities have changed, then it may be time to weigh new options. As she considers transferring, it's important for her to think about long-term career goals, and to try and plan on the steps that are necessary to reach them. If she decides to make a change, I would encourage her to work closely with the counselors at both schools to facilitate a smooth transition.
Make every effort to keep the lines of communication open. Encourage her to talk with you about her feelings and concerns and be careful not to be judgmental as she shares them with you. I know that you are concerned and only want the best for your daughter. The key is for her to reach her potential.