Actually Dave, I think you are better off with the late seating if you can handle it for the Alaska cruises. You are in port late and you would have to cut your stay short (at least we did) to make it back to the ship in time for dinner. The sun really never goes down so it messes up you body clock anyway. I'm not surprised that the early seating is full since with the time changes, many people think they need to eat earlier. But your body will adjust.
I'd go ahead and try to have the head waiter change it for you and if he said he couldn't, then I wouldn't worry about it as that might work better for you anyway.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007