I was invited on April 10th to configure RWD. The AWD is still locked for me in the configurator. My guess is that a lot of employees and Model X/S owners were lining up for that option. If the availability does not start until July, I could see many of the "already invited" to wait a month or two, to get the AWD invite.
Seems like only one round so far. Doesn't appear to take into account if it is prior owner as many non-owners have received AWD config. Elon stated it would be based on time you placed the reservation.