Have aliens taken people over as walk-ins in the same way a human can take over a living body if the prior soul exits?

They can take over a human body through controlling the deep subconscious and then moving upward in the levels of the mind in maintaining that grip and subverting the ability of the human to exercise their free will. This takes place in the same way that the spirit meddlers can produce the phenomenon of sleepwalking. While the conscious mind is switched off during sleep, the spirit can take over the body in some individuals and carry out quite complex physical operations. The extraterrestrials can do this as well, and this can serve them in many ways to carry out clandestine activities, and subvert human plans by taking over individuals and gaining access to secret locations and the inner sanctum, so to speak, of institutions and government for many nefarious purposes. This is simpler and less complex to arrange then a replacement by a Reptilian shape-shifter, so it is used for short-term control and serves them in many ways.