- Edited
KoraL Just a brief explanation regarding reincarnation, especially since it's an Asian concept and some misunderstandings may arise from it.
For most reincarnations, you don't simply replace an existing soul. In Buddhism, for example, the belief is that when you die, your soul continues to exist and usually exists somewhere (in Chinese mythology, you go to the underworld and wait in this super-long line to drink some soup/tea of forgetting brewed by Grandma Meng after crossing a bridge, and then you get reborn). It's more of a rebirth, where your soul is reborn in a new body, a new family, new time. You don't simply come in and replace an existing soul. The soul is eternal and indestructible, the physical body is not. Many religions state that reincarnation occurs many times - your modern life today could possibly be the hundredth time your immortal soul has been reborn and living in the material world. In Budhism, you get reborn and reincarnated again and again until you finally attain enlightenment and go to Nirvana - that's the goal of all practitioners of Buddhism. To go to Nirvana and break out of the cycle of death and rebirth.
The existing soul being replaced is transmigration. Usually what happens in most stories is that when the main character transmigrates, it's usually into a dead body. The previous soul in the body has gone to the underworld because he/she has died. In many stories, it's simply a reawakening - a rude shock to the head, and you suddenly regain memories of your past life (this is basically reincarnation, but skipping forward so that we don't have to go through the whole baby and child stage, which can be boring and tedious). But if you're worried about the replacement of soul, that soul usually just goes to the underworld or afterlife because it works on the premise of he/she died, and this random assassin/marine/soldier/martial artist/genius from another world also died and his or her soul decides to go into the dead body and reanimate it for some reason.
So you don't really need a society or universe at all. I hope this explanation helps.