Boeing has been building and testing 737 Max jets throughout the grounding so having them flying is not a great surprise.  However, with the grounding order lifted by the FAA, things are moving into a higher gear.  United took delivery of a jet and American Airlines has indicated it will start service before the end of the year.  Two jets were up on the same day which leads me to think that they have already undergone the mod programs and are being tested prior to delivery to the airline.