I've already completed some Upper Division Cluster course work, but have decided to change which Cluster I want to use to fulfill my University Studies requirement. Do I have to start over and take twelve credits of courses for the new cluster?

Yes. All twelve credits of Upper Division Cluster course work must come from the same cluster. Many clusters cross list classes, and students may be able to find a new cluster that can utilize some of the course work already taken from the cluster with which you began.