The formal end of the Regency begins one second past midnight on the day of Gregor's twentieth birthday. All of the Counts and Ministers are in attendance at Vorhartung Castle and they watch as the Lord Regent kneels before Gregor and places his hands in his. They speak the formal words of the oath and then the Counts and Ministers come, one by one, to renew their oaths.
It takes about two hours. Barrayar is no longer governed by a Regency government, but Gregor is as much of a puppet today as he was yesterday. Everything's changed, but nothing's different.