no it is not too late - you can add the dining plan up to 72 hours before arrival day so upgrading would also be allowed
...but you may be charged an extra $50 to make the change at this late date. Used to be that they never charged the change fee to upgrade (because you were paying them more, lol) but there have been reports lately that people have been charged the change fee even for upgrades. We were never charged, so maybe you won't be either (have upgraded both room & dining plan at less than two weeks out and no extra fee.)