What are peoples thoughts on meetings booked before Aug 1st for early 2019 ?
Will they honor T&C at time of booking, or are you stuck with the earning rate from the new program?
The present terms and conditions indicate it is the terms in place at the time of the event, not reservation, which govern. A few people have reported someone from Marriott indicating the old rules will apply after the merger for meetings booked before the merge date but these seem more akin to having a friend's sister's cousin's hairdresser's neighbor's cat sitter making a promise than an actual policy statement. If this is important to you, reach out to the hotel or Marriott and ask for written confirmation that the legacy parameters will be followed. Waiting until afterwards to argue that you thought the old rules would apply would likely be a losing argument.
And again, there is no confirmation the rules will be changed. My advocating taking prudent measures to cover yourself should the internet rumors be correct and the rules change is in no way intended to be a statement that the rules will definitely change.