Sorry to disagree, but the least likely reason for them to upgrade you will be because some of the more expensive space is unsold. This would be unfair to those who paid for it... and is not likely to happen, on Cunard or any other line. More likely is that, close to the sailing date, it is difficult to sell the more expensive cabins and they will sell and resell and resell the less expensive stuff and keep moving people up. You might get a better cabin in Mauretania... or Caronia... but don't expect to go from Mauretania to Caronia... or to be upgraded into the Grill categories... it almost never happens and for good reason.