It depends....we've done several B2Bs and each was different. You get a sheet of instructions for YOUR cruise in your cabin on the last full day.
If you are staying in the same cabin, you do not have to pack your stuff or do anything that way. In Port Canaveral, we had to get off the ship at 9:15, go thru US Customs, walk back into the terminal and check in again. We boarded the ship before the terminal opened to newcomers.
In Barcelona, we had only to go to GS to get our new KTTW cards--we could stay in our room or whatever.
We suggested that our room host not make up our room on the transition day since it is such a busy time for him/her...he did it anyhow.
Food/beverages are available somewhere, usually in the Cove Cafe. In Barcelona there were so many families doing B2B that the Cove Cafe was for adults and there was food/drink in the Promenade Lounge for families. After all, breakfast ends at 8:30 and lunch doesn't start till 11:45...one could hardly expect to not be fed on
DCL!
The transition is a great time to take photos without other people in them and enjoy "your" ship.