Think it depends on what kind of room you have. If it's a standard view at the CBR, you're probably going to get a room. If it's a lagoon view concierge at the Yacht Club, you're probably going to have to wait. Everything is possible, though. We've been 2 for 3 on early check in, with 11:00 being the earliest we've tried.
Someone once posted that upgrades are much less frequent for early check in, which makes some common sense, but is a very interesting question. It'd be great to hear people's experience on that.