The last time I sent in a corrected form with new flight numbers to ME on June 3, this is the reply I got:
Unfortunately, we do not have the ability to create and mail your confirmation letter and luggage tags before your arrival date. You are still able to utilize all of the services that Disney’s Magical Express provides. Please follow the information provided in this confirmation email in the event your collateral is not received before travel.
My arrival date is Aug 15.