If the hotel will be completely sold out, then it's not very likely they will be able to upgrade you to a better room. The upgrade is to "best available room", but if everything is already reserved they'll give you the room you booked for your stay. Rooms are only available for an upgrade when they haven't been reserved by someone willing to pay for the upgraded accommodations.

I have been upgraded before check-in, but that is only when the hotel knows they'll have rooms available. Again, it the hotel is full, they won't promise you an upgrade "pre-checkin".