What they did is illegal. If it goes down would they lower the rate? A hotel should not change the rate when you get there. This is basically price gouging since you were desperate for the room at that point and stuck. Besides the canceltation penalty had passed. They didn't take any steps to notify you either.

I would send a letter to GM asking for
(1)A cease and desist of doing this to everyone affected (not just you)
(2) Refund of $6
(3) Some type of punitive damages be it 10% off next time or points or a search of their records to refund $$ to everyone overcharged as they collected $$$ off people who didn't have the time to complain.

I mean changing rate on you and others is a classaction offense. When you book it did it say they can change rate on you?