in possession, the spirit remains foreign. so let's say the spirit of jack possesses the body of jill, then by definition jill takes on the identity of jack, even though she looks like jill. what defines the person is the spirit, because traditionally, spirit is thought to be a collection of self-identity, memory, and way of thinking that is unique.

if you apply this relationship to john and elijah, it just doesn't work out. if john the baptist is entirely elijah, then he should act, talk, behave, think like elijah, with no identity of his own, and should not identify himself as such, nor should the bible identify him as an entirely different individual.

therefore, in order to say that john and elijah are one and the same, yet retain 2 separate identities, the only way would be reincarnation, which demands that the spirit shed its old identity and takes on a new one altogether. otherwise, john and elijah will have to be entirely different individuals, despite their similarity in ministry.