You can use points at check-in or during your stay with the "Instant Redemption" program.
Here is a chart of the redemption rate:
Listing of the Awards required to redeem specific dollar values:
|Points Required||Up to Dollars|
Personally I would change your existing reservation to points as the redemption ratio is not very good in this program. Your points would also go further by ordering Marriott Gift Cards or Cheques. Example you can order a $200 Marriott Gift Card for 44,000 points and in Instant Redemption you would have to use 59,000 points to get $200.
I apologize for not getting to your response before your stay. In the future I would advise contacting reservations to switch your room over to "redemption". If the room type is not available using points, I would suggest using gift cards like vaboywndr has suggested.