Again, knowing now that you are looking for a cabin midship, your odds are now decreasing again. Those fill up first because everyone thinks there is less motion in the middle of the ship. I wouldn't worry about it too much if this is your reason for wanting to move. The size of the ships we cruise on now makes the difference in motion between the midship cabins and those on the ends not truly noticeable. You have to be in seas that are pretty bad to be able to a difference and if that it that case, no matter where you are located, you are going to feel motion.
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, Magic 1/14, Freedom x 2 6/14
NCL Dream October 2006, RCCL Mariner OTS June 2007, Celebrity Silhouette 2013