Tesla states on the order page "Delivery timeframe will be based on reservation date, order date, delivery location and vehicle configuration." We don't know exactly the algorithm Tesla uses behind the scenes, but it seems existing owners no longer get priority. Some forum posts around the time of the initial dual motor Model 3 go-live confirm the change in policy, at least with respect to what the customer service reps have told people that called in to change their orders from RWD to AWD.