Originally Posted by Horace

You don't need to cancel and rebook. There's a better, safer way:

1. Log in to Marriott.com on a web browser.
2. Click "My Trips" from the options at the top.
3. Click "Expand" for the reservation you wish to check/change.
4. Click "VIEW/MODIFY" for the reservation you wish to check/change.
5. Note how many points your current reservation uses.
6. Click "EDIT ROOM"
7. See how many points the same room type would now require.
8. If the points are lower, click "UPDATE" (adjacent to the new points) and follow the subsequent instructions to complete the update. The difference in points will be credited back to your account almost instantly.
9. If the points are a same or higher, click "RETURN TO ORIGINAL RESERVATION."

Your original reservation remains "safe" unless you follow through with the update. This is better than canceling first. If you cancel first, there's always the chance that you won't be able to make the new reservation and you will have lost your original reservation.