This is probably the best advise though in reality if you try to call the hotel, they may route you to Marriott's general "meetings & events" team. They will tend to be less flexible and not negotiate much.

I've found that Marriott's Meeting & Event team tend to be more "flexible" if you are purchasing not only rooms but conference space and catering. That is where they really make money and where they will start working with you.

If you really ONLY want rooms and nothing else, I've found other chain's
"event" coordinators to be more flexible than Marriott's. In fact, more than a few times I was able to find cheaper rates (with much less restrictive cancellation polies) by just reserving the rooms through the Marriott website. The Meetings and Events team will want you to sign a contract and have fairly draconian attrition clauses, etc. So be careful if you go that route.