Also the system runs software which allows for updates and modifications. Easy to reconfigure the individual satellites and system. There is constant communication with the constellation for health and other details.
When my co-author and I spoke at AF Space Command in 2016, we got to see 2 SOPS where they control GPS.
http://www.gpsdeclassified.com/easton-and-frazier-speak-at-peterson-afb/