During the Punk Hazard arc, Trafalgar Law swaps Sanji into Nami's body and Smoker into Tashigi's. In spite of them being in bodies not addicted to nicotine, it seems they both still need to smoke. Nami (in Franky's body) even offers Sanji a pack of cigarettes saying something to the effect, "You need them, right?"
When Trafalgar Law swaps everyone, the image shown is some weird heart thing, not swapping their brains. I assume this is some kind of representation of a soul, but if they still need nicotine, maybe I'm wrong. So, what exactly does Law swap? And why do Sanji and Smoker still need to smoke?