I have done several back-to-back cruises on Carnival. They give you a letter in your stateroom with instructions on where to meet on the morning of debarkation. For us it was like 9:30 am or maybe 10:00. We were escorted off of the ship by a staff member from Guest relations and then taken directly to the next available customs agent (bypassing the hundreds of guests in line) who just waved us through. Then, when we returned to the ship we were issued our new sea pass cards and a new photo was taken.
The whole process took less than 15 minutes and it was really fun watching everyone leave the ship, knowing that we got to stay for another cruise :-)
Good luck and enjoy!