The best explanation of who is getting "Grandfathered" into the new unnamed program is from the Points Guy:
Who's Grandfathered into Marriott's Lifetime Premier Platinum Status?
I won't even go into how "grandfathering" is such a sexist term.........and how poorly Marriott has explained this. lifetime platinum premier elite
Actually, this is another instance where 'The Points Guy' is not quite on point, and I commented on this - the very 1st of the 4 groups is incorrect (and you can read the comments to see this). Current (or now former) legacy Marriott LTP members pre-8/18 were grandfathered on day 1 (whichever day you want to count as "day 1" after the dust settled) and do not have to wait until 2019. All other legacy (group #1 from your referenced Points Guy page) Marriott members who reach LTP after 8/18 but before 1 January 2019 will be grandfathered into LTPP in January.
The LTPP criteria is no different from what I've stated (as well as others) on Insiders previously.