These characters have transmigrated into someone else's body. They may have died, or this might have occurred due to magic or the power of a deity or other supernatural being.
These couples start out hating each other/being on opposite sides of a conflict, only to fall in love by the end of the story.
Similar to reincarnation, transmigration is when the protagonist takes over the body of a person existing in (usually) another world. It is sometimes also used to refer to being transported back in time (in such cases where the...