A soul “walk-in” is a higher soul (other than the original soul) that takes over, as result of a deal, the body of a lesser soul. The lesser soul returns to higher dimensions (to God if you want) while the higher soul uses the body for a new destiny. Certainly, a body can be taken over also by a smaller soul than the original one in which case the term is “possession”.
How does the walk-in soul manifest
The mind that was taken over by a walk-in soul will feel something changed a lot, but from my experience it won’t be able to identify exactly what. It will think something in religious terms since these are the ones readily available, like an archangel is now next to him, a prophet or another high saint helps him now directly, and the like. To understand what happened I observed that one needs a mind/body complex at east 4.5D vibration to have even a hint of what is happening, lots of practical spiritual experience (not religious), and also confirmation from an independent and trustworthy source.
Walk-ins souls and enlightenment
Many spirituals think that a walk-in soul is necessarily enlightened. This statement doesn't hold true in my research of such souls ...
Enlightened becomes the soul, not the mind. The mind is just a vehicle for the soul, a prop, an instrument for enlightenment. The soul is eternal while the mind turns to dust at the end of life. The mind and body of an enlightened soul you might call it “wise” if you want a name for it, since it finally reached 100% the understanding and emotional conviction that everything is related, nothing is separated.
To take over a body as a result of a deal it’s not at all necessary that the new soul to be enlightened, it can be anywhere as vibration between 5D and 7D, and in very few cases even higher.
“Walk-ins” are generally higher souls with a specific and precise mission in that life, that’s why they accept the risk of descending in such low vibrations / dimensions as ours. Many times I saw they sub-evaluate the risk of coming here. They lose compass due to 3D emotions and become just ordinary souls when they lose control of the body they use.
True walk-ins are just a few thousands currently on Earth, but many other cases are mistakenly considered as walk-ins while being something else.
The soul can be enlightened now, but can also decrease in vibration if the mind takes less inspired decisions during life and lets low-level emotions invade it. I saw this personally quite a number of times in enlightened souls that worked tenths of years with themselves to reach enlightenment, and then reverse to square one.
Drunvalo Melchizedek is a good example of a walk-in soul, but I know a few dozens others.
Technically, Jesus Christ is a walk-in soul when being baptized in river Jordan, another is Ramakrishna in the 19th century, and many others higher souls.