Ibur Neshamot - More than one soul inhabiting a single body
There is another kind of gilgul (reincarnation) that can take place while a person is still alive. The ARI calls this form of reincarnation, Ibur.
It is usually thought that gilgul takes place after a person passes from this world, after the death of the body, at which time or soon after the soul transmigrates into another body. Ibur does not work like this. It involves receiving a new (higher) soul sometime during one's lifetime. That is, a new soul comes into a person's heart while he is still alive. The reason this is called Ibur, gestation or pregnancy, is because this person becomes "pregnant" with this new soul while he is still alive.
This phenomenon is the deeper explanation behind certain people going through drastic changes in their lives. They either undergo a change of mind about certain things or change their lifestyle, and thereby ascend to the next spiritual level. This is also included under the general heading of gilgul-incarnation because they are now hosting a new soul [or an aspect of their own soul or a higher soul of which they are a part] in order to be a vehicle for that soul's rectification. This is what occurs when a person is ready to advance in his soul evolution. This is why the soul has five names, each higher than the other, nefesh, ruach, neshamah, chayah and yechidah.
According to the Zohar, the four higher levels of the soul usually enter a person during his lifetime in Ibur: First, a person receives nefesh when he or she is born; then, when they merit it, they receive ruach; when they merit it, they receive neshamah; when they merit it, they receive chayah. The higher the level, the rarer its occurrence. Very few have ever merited to neshamah, let alone chayah. Nobody has ever received the highest level, yechidah. Adam would have received it had he not sinned.