Usually, but in extreme load cases in a particular class, no. For instance, if Y is zeroed out, you don't know if that's -3 seats or -50 (which is the case in some Asian stations at the moment!). Actually if it's -50, they start to close out J and F too, but you get my point - EF might show availability, but that cabin may in fact be overbooked and they are going to op-up people instead.

You can upgrade with miles or cash at the airport yet, if the controller allows it (it is usually available on a space-available basis, but even then, controllers may be expecting few no-shows are are overbooked, even though space may look vacant on a seat map). There aren't proactive upgrade offers in the vein of BA, but sometimes if there is mileage availability they offer it there as well.