We've done a back to back 7 day. We booked the same stateroom for both trips. We did not have to pack up. I think they change it at times, however, we were escorted off, checked in and were escorted back onto the ship before they allowed the other guest on. It was a blast. We were on the ship, with only cm's, for about 3 hours before we had to check back in. I did my laundry and we walked around.
I think if you change staterooms, obviously you would have to pack up!