In my opinion, they treat Diamonds very well here, and one might guess you will get an upgrade unless house is full and they just don't have one to give. That said, if I was in your shoes, I would call a week before my arrival and talk to a FO manager (maybe start with some other property questions, just to make it a friendly(er) discussion, and then noting I'm a Diamond ask what upgrades upon check in might be available and discuss the blocking or my room) - as opposed to just leaving it to chance.