Have many higher selves had physical lives in the past? Once they become a higher self, do some still return to physical incarnation for mission lives? Can a higher self become a walk-in and replace the soul they were acting as a higher self for?

The higher self is a soul extension but less a separate integrated being than an incarnated person. There is always a tether, the so-called silver cord that is a metaphor representing the interconnectedness of consciousness, so people in physical incarnations are nonetheless still connected to both their higher self and the deeper soul level. This is much more the case for the higher self, that it is still an integral part of the soul but simply a kind of compartmentalized aspect with a highly focused mission—to be monitoring and interacting with the incarnated soul extension, to be a resource and a safeguard as well as a source of guidance, inspiration, healing, and protection. This is a sacred duty and in a sense reflects that of the soul itself, which takes quite seriously its sacred duty to be a member of the community of souls and be of service to the divine and the Divine Plan. As such, the higher self enjoys ready access to the expansion of divine wisdom always ongoing, and nowhere reaches the level of depravity and darkness of living human beings reacting to the challenges of physical existence and meeting with harm at the hands of others, and so on. So the higher self, by its very nature, is a lofty part of the soul and needs to be available to be called on for such duties, and that is its purpose. As such, it has no interest or desire of its own to incarnate anywhere because it is totally dedicated to carrying out its functional purpose, so the higher selves do not trade off with the parts of the soul that incarnate as physical beings to carry out such roles. After all, if you think about the difference between the duties and the respective environments and the contrast that exists between what humans experience as a physical entity and what the higher self enjoys, being within the divine realm, these are extremes of polar opposites that are inherently incompatible. Only those aspects of the soul expressing great courage and a desire for novelty, challenges to overcome, and bold adventures, comprise the attributes that are a basic necessity to mount a physical extension of the soul itself. This would not be undertaken by the more lofty and fully developed parts of the soul sharing in a more profound way the sum total of soul attributes. It is not a complacency, it is a state of completion they enjoy, those aspects of consciousness within the soul that already have reached a level of existence that is quite unthinkable to surrender, even for a short while. There are other parts more uneven, more restless, more volatile, and it is those giving rise to a desire to take on the adventure of physical incarnation. So it is not a question of one being more worthy than the other, but simply having a different makeup and having a good sense of what that means and what it can lead to, and sticking with what is most appropriate for one’s being, because the other choice would represent a lessening from its perspective rather than an opportunity to grow further in some way.