As Dave noted the main difference is the Fascination has had all the Evolutions additions while the Elation has not. The two main areas you will notice is the Lido Deck pool area and the Waterworks on the aft Verandah deck. Another difference in the two ships is the Fascination has had new balconies added. Where on the Elation you will have to book a Suite to get a balcony.
For the second half of the B2B you will be instructed to gather in a designated area without luggage. A Guest Service Rep will escort the B2B guests off as the last debarking group for customs and immigration. The Guest Service Rep will then escort the group back on board the ship to check in at the Guest Services Desk and issue a new S&S card. Once you have the new card you can come and go just as if you were at a port of call.
If you have the same cabin for the second cruise you can leave your luggage in the cabin. If you don't they will send someone to move it to the new cabin for you.
Now these are the normal procedures, but they can vary between the embarkation ports. I think (not positive) that LHP recently did a B2B on the Elation. Maybe she will know for sure how it works specifically for Mobile.