My sense is that the status benefits that are outlined in the existing marketing materials will have to be honoured for the status year and Marriott will only be able to change those benefits with sufficient notice.

In other words, the benefits associated with SPG Platinum for 2018 will have to be honored now that the qualifying period for the 2018 status year has begun. Marriott will likely have to communicate changes, if any, by the third quarter of 2017 for the qualifying period commencing on Jan 1, 2018 for the 2019 status year.

This would likely be the least messy option Marriott has to ensure that they don't create unnecessary issues by proceeding without sufficient notice.

Of course, the best way to avoid issue would be to change the benefits and market them as "improvements" but from what we've seen so far with the Marriott/SPG account linking and conversion rate, Marriott seems intent on preserving the value in their acquisition of Starwood and the SPG program.