Also noticed this recently with my reservations. My inclination is that its a United-side change. When I go into a reservation and click change, I get the same message above. However, if I go into the reservation (typically single cabin trips), click "upgrade", and then cancel out of the upgrade screen, it brings me to an older version of the United.com page that allows me to make changes. Haven't found a workaround if I'm on a trip that can accommodate an upgrade.