I had 3 non-refundable rooms at the Hilton Garden Frankfurt and as it turned out, only needed two. Called well in advance and they gave me credit for the cancelled room's cost to use for the restaurant and bar.

Also, I made a mistake like OP with the Hilton Leningradsky. I saw what I had done immediately so I called Hilton Honors immediately. They were able to call the hotel and get it cancelled, perhaps because it was done so quickly.