From the Marriott Rewards FAQ page:
Q: Are there any changes to the RewardsPlus program with United MileagePlus?
A: There are no changes to the
United RewardsPlus offering. If you have Platinum Premier Elite status, you will still be able to link and upgrade your United MileagePlus Account to Premier® Silver status. In addition, all members receive bonuses when converting points to United MileagePlus miles and on RewardsPlus Hotel + Air Packages.
This indicates that you will need to be Platinum Premier (75+ nights) on the new Marriott side to get United Mileage Plus Silver status. Thus Marriott Premier Platinum (75+ nights) would obviously not get United Gold, as 75+ nights is the cutoff just to get United Silver. Therefore, best case would be that Ambassador gets United Gold, although that obviously is not specified.
Also, it is not specified what benefits someone with United MileagePlus status but no Marriott status will now get.