Cruise lines affiliated with CLIA (Cruise Lines International Association) will immediately adopt a new muster drill policy. The drill must now occur prior to departing port. For those who board late - when the drill is already in process or completed - they will will have to be provided individual or group safety briefings 'promptly', which I assume means without reasonable delay although probably doesn't mean prior to the ship leaving.
It already has taken affect. This is no different than what has been the practice for ships leaving U.S. ports for the past many years. The drill happens prior to the ship leaving the embarkation port.