Well, the T&Cs imply that a stay booked that far back wouldn't count for the elite bonus, but not because it was booked before Sep 2 (that in itself is not a problem), but because it was booked before registering for this promo.
sdsearch
To reconfirm - I was specifically told that MR members will earn the 1000 points plus the 500 elite bonus for all stays completed after registering for the promotion regardless of when the stay was actually booked. I am confident of the information I was provided and don't plan on rebooking any of my stays.
Regards,
GoingGal