When a walk-in takes place, it is not because the entity has left temporarily, but, like death, has left permanently. This occurs when the entity essentially wishes to commit suicide, and would do so if the walk-in were not allowed. The spirit guides are aware of this, conferences take place, and all is agreed. Since this changes the nature of the incarnation agreement, it is like an amended incarnation agreement, terminating the incarnation for one entity, but instituting this for another. Walk-ins are very rare, although they get a lot of press.
Possessions are one such instance, and exorcism is where the original entity is persuaded to return.
Another example of a walk-in is where there is extremely important work to be done, but the incarnated entity does not feel up to it. This occurs only where there had been conferences between the entity incarnated and the entity desiring to enter. Also present in these instances is a mission that both feel is important. One should not assume that disincarnate entities are waiting around for an opportunity to jump in, or that this would be allowed by the spirit guides, in any case. As we say, this is an extremely rare occurrence.