Assuming I book using a fully cancellable rate which doesn't have a minimum number of nights, what exactly are my rights? I'm thinking of Best Flexible Rate, Friends and Family, flexible partner rates.

I've never had a problem deciding to check out the next day and shorten the stay, but was this a right on a flexible rate, or was the hotel extending a courtesy? For that matter, if the upgrade at check in left me underwhelmed, could I decide immediately to shorten the stay to one night without penalty?