We are booked at Pop for 12/11-12/15, in a standard room but needing a preferred. I thought that calling on the hopes that someone would cancel would be the thing to do, but I was finally told very specifically that only a certain number of rooms and room types for each discount type are available -- once they're gone, they're gone. And that even if someone holding a preferred room with the same discount code as us were to cancel, that room would go back into general inventory at full price, and not be available to us with our discount code.
I personally think that stinks, and would LOVE for someone to tell me that that CM just didn't know what she was talking about!!