[Content Warning - death, injury, possession] walk in

A “walk in” occurs when the original spirit attached to a body leaves, and someone else occupies and uses that body. Explanations of this in the 70s frequently made mention of the change of spirit (or soul, if you prefer) being associated with a near fatal accident, but that is not always the case.

The new occupant may have a markedly different personality, and live life in a very different way – again, in the 70s this was often described as “more spiritual”.

This is not a case of possession or obsession, as the original owner of the body has freely given their consent for a permanent change.

Probably the most famous (or infamous, depending on your perspective) case of this is that of Lobsang  Rampa, who described his experience of transmigration, as he termed it, in "The Rampa Story", where the soul of the Tibetan Lama Lobsang Rampa took over the body of an English plumber, according to Rampa's description.

There have been books about this as well, such as the Christian biased Ruth Montgomery's "Strangers Among Us" (unfortunately, my copy is one of the books which has disappeared over the years, so I can't provide publishing details and an ISBN, but the Wikipedia pahe on this author credits the published of this book as "Coward, McCann & Geoghegan", and the year as 1978). 

This is also something that I have some personal experience with, as I went through something similar, although technically more obsession than possession, when I was in my 20s and struggling with a range of issues (none of which was helped by the ignorant discrimination and prejudice of society at that time - and ignorance is, in my view and - arguably - that of Buddhism, truly a spiritual crime).

There are a few other books and films at the Wikipedia page on this topic.