In these situations, if rates have increased and rebooking is not an option, you are really at the mercy of the local hotel. Accor reservations isn't going to modify your reservation, by dropping a night and holding the nightly rate. AFAIK, only the local hotel can do that. Of course, they don't have to do that, so if they say 'no', then there really is nowhere else to escalate that to. While they're not obligated to, I've always been successful getting the in-house Reservations Manager to cooperate on such requests at the hotels I stay at, as long as I wasn't on some sort of promo that required a minimum number of nights, or a 'book 4 nights pay for 3' scenario, where my existing rate would never have been valid on the shortened number of nights.