I stayed last week at the Roosevelt, one night paid and one night certificate. The Diamond Desk operator made the certificate reservation. The operator put notes in the reservation that I had a paid night and would like to stay in the same, hopefully upgraded, room. That part is up to the FD staff at the hotel, and they came through beautifully this time. I was upgraded, they set up the key so that it worked both nights, and housekeeping did not expect me to be checking out. (3 years ago I had the same scenario at the same hotel, and though the FD person tried to do that, the key did expire and housekeeping did knock on the door.)