If you would like to know the full details of everything you can contact KCL Residences
Nothing happens with your deposit. The purpose of a deposit is just in case you cancel your room altogether, they won't refund you. Nothing happens to it when you swap rooms.
The biggest reasons people swap is either costs (they want a cheaper accom) or distance. I just want to be closer to my campus, so mine would be distance. What about you?
By the way, the form for the swap opens at Oct/Nov.