She popularized the walk-in theory.

George Hunt Williamson (1926 – 1986), aka Michael d'Obrenovic and Brother Philip, was an American flying saucer contactee, channel, and metaphysical author who came to prominence in the 1950s.

Williamson became a more obscure competitor to George Adamski, eventually combining his own channelling and the beliefs of a small contactee cult known as the Brotherhood of the Seven Rays, led by Marion Dorothy Martin (1900–1992) ["Marian Keech", Sister Thedra], to produce a series of books about the secret, ancient history of mankind: Other Tongues—Other Flesh (1957), Secret Places of the Lion (1958), UFOs Confidential with John McCoy (1958), Road in the Sky (1959) and Secret of the Andes (1961). https://en.wikipedia.org/wiki/George_Hunt_Williamson

A walk‐in is also the name popularized by the writer Ruth Montgomery for a soul who enters a fully grown body without being inconvenienced by the birth process.

Ruth Shick Montgomery ( 1912 –  2001) was a journalist with a long and distinguished career as a reporter, correspondent, and syndicated columnist in Washington, DC.

Later in life she became a self-proclaimed psychic and author of numerous books on occult and New Age subjects, whose predictions regarding the "polar shift of 1999" heads a long list of attempts to foretell the future. She was a biographer of alleged paranormal medium Jeane Dixon and a protégée of Arthur Ford, who posited unsupported claims that he, like Edgar Cayce, could access the Akashic Records (or database) of the Universe.

Montgomery, who enjoyed great financial success via her prolific New Age writings, believed that mental and physical illnesses often have their origins in past lives, birth marks are the sites of past life injuries, and commented that often children born with serious defects or illnesses are in fact repaying debts incurred in previous existences.

Montgomery is particularly noted for popularizing the walk-in theory whereby a person's soul can depart a hurt or anguished body and be replaced with a new soul which overtakes the body. She filled a book with an extensive list of present-day and historical individuals she said were examples of "walk-ins" and claimed several US presidents were among this group. https://en.wikipedia.org/wiki/Ruth_Montgomery