We did a B2B in 2008 and did have to get off and reboard. Everyone doing a B2B met with a crew member by the excursion desk in the main entry lobby and we were escorted off the ship. The checked us out and checked us back in, then we went down through the customs area and reboarded. The entire process took less than 30 minutes. We did have the same cabin for both cruises, so we just left our stuff in the room. If you don't have the same cabin, however, you obviously have to repack.
They have changed the process on occasionI know that for a time you didn't even have to get off of the shipbut for the past few years you have had to disembark and reboard.
It was really cool to be able to wander around the ship with no one else on board. They did let us check out the big suites and go pretty much anywhere. You should see the looks you get from people waiting in the port while you are looking down on them from Deck 10 or when the "first" family boards and you are there waiting for them.