Email from Marriot reward stating the same thing, both program will run in parallel for some time after the merger:
"After we become one company, we expect to run parallel loyalty programs while we engage in the complicated work of integration. During this period, there is no change to how you manage your Marriott Rewards account or book reservations, and you will maintain your existing member benefits for some time.
The good news is that we have many possibilities ahead of us because Marriott and Starwood both have great loyalty programs. We intend to draw upon the very best of both Marriott Rewards and Starwood Preferred Guest® (SPG®) to provide even more value to our members."
Sounds like a new plan is in the work, until shareholders complaint about earning then devaluation & cutting benefits start...