It works... if the hotel has this option. Did it with a reward stay I screwed up a few months ago.

As for the OP "I am never staying at Marriott again" I would also suggest not staying at Hilton or IGH as I know both of them do the same thing. If you book a reward stay and don't show they charge you money. (Actually I expect that staying home is probably the only option as if you don't show and don't cancel hotels world wide charge)