Yes ADS-C is part of Future Air Navigation System 1/A (FANS 1/A) that also includes CPDLC and ACARS. The system was put into use initially by Boeing in the late 1980's so it has been around for around 4 decades. The ADS-C contract can be with up to 5 ground stations world wide. The ground stations as you say can re-contract without the pilot even knowing and request other position reports or reports on level changes and leveling etc etc.
ADS-C is being extended in Europe and USA to use VHF Data Link (2) (VDL-2) and will be used to control aircraft flying business (user preferred) trajectories. So one of the new messages will be Extended Projected Profile (ADS-C EPP) a series of up to 128 waypoints and pseudo waypoints ahead of the aircraft with their associated attributes/constraints such as speed, flight level, time. (Pseudo waypoints are for example Top Of Climb, Start Of Turn). Again the ground can ask and your FMC will reply you don't even know it has happened. This is all part of ATN-Block 2
Hey but don't stop the ICAO meetings and standards people re-inventing things the avionics salesmen love it.
