Keep printouts, email confirmations, and any other documentation. Marriott offers a room at a certain rate, you decide to buy it. Doesn't matter if it's for cash or points or in trade for a bunch of bananas. You're accepting their offer. How are you to know they aren't having a sale on suites?
This doesn't mean they won't try to back out of it, but if they do, you have pretty solid ground to stand on to demand they make up for their mistake that "ruined your vacation". Perhaps you settle somewhere in between a standard room and the Presidential suite if you really feel you won't get what you booked.
If it were me, I wouldn't bring it up proactively. I understand the fear of screwing up your New Year's Eve plans, but I'd take the risk. But for me the fun of (non-work) travel is where you go and what you do, and I try to spend as little time in my hotel room as possible.