I just had the following chat with MR which confirmed:
You have the rest of 2018 to meet the original Lifetime Platinum requirements to be grandfathered into the Lifetime Platinum Premier
Ashley B (4/18/2018, 2:15:25 PM): You have been connected to Ashley B. How may I help you?
Me (4/18/2018, 2:15:41 PM): I had a question on lifetime status in the combined program
Me (4/18/2018, 2:16:52 PM): If one qualifies for lifetime Platinum in Marriott Rewards under the "old' program rules (750 nights + 2 million points) after August 1 and before end-of-year 2018, would they be grandfathered Lifetime Platinum Premier or are only those who make lifetime platinum in the current MR program by July 31st, eligible to be grandfathered into LT Platinum Premier?"
Ashley B (4/18/2018, 2:18:04 PM): That is a great question. Let me look into this for you!
Me (4/18/2018, 2:20:19 PM): thank you
Ashley B (4/18/2018, 2:22:35 PM): We are just double checking this for you. So that we do not give you an erroneous information. It will be just a couple of minutes.
Me (4/18/2018, 2:22:59 PM): thank you
Ashley B (4/18/2018, 2:30:23 PM): I apologize that took so long. I have confirmed with a supervisor that you have the rest of 2018 to meet the original Lifetime Platinum requirements to be grandfathered into the Lifetime Platinum Premier status.
Me (4/18/2018, 2:30:38 PM): awesome, thanks great news, thank you!
Ashley B (4/18/2018, 2:30:50 PM): It has been my pleasure to chat with you. Is there anything else I can assist you with today?