I’d consider it a very advanced variant of possession, requiring not only a deep possession of both people by each other, but also a way to get the people out of their bodies permanently.
Due to the golden rule on this planet of free will, you can’t do even a mild possession without permission, and you need this on both sides.
Add that as a mage you have awareness and a repulsion to the idea, and it’s a non starter. Anyone wanting to do this will find much easier pickings with millions more people than you. You own your body and have the higher ground, full control and full integration between your spirit and body, which they don’t, so it’s takes almost no effort for you to say no and the fight is already over.
I think you basically both have to die and then do a “walk in” to each other’s still living bodies.
So I don’t think you need to worry, specifically because you said this:
You don’t give your permission, end of story.
I don’t think people were short because they saw the question as unimportant, I think they just don’t know, as no one’s seen it. 'Nigh impossible" is a fair description of the statistic given that nobody’s seen it or knows how to do it, that we know of, and us lot: we read a lot 
Even walk ins are very very rare.
I am aware, I think, and this is a purely UPG thing on my part, of one mage that intended to never die by walking in to other mages in an unending cycle of forced possessions: and all that’s happened is he’s basically become a parasite, feeding off a few mages at a time but never really getting a body of his own.
He is dying, he’s already a “lesser entity” and will fade to a mindless daemon and then dissipate to sleeping in the land and then nothing if he can’t get out of the astral. He needs to incarnate but won’t as he knows he’ll forget who he is, so that’s a kind of dying too. He may not be strong enough for a human incarnation any more anyway.
So it’s not so easy, and that’s why we don’t see it. If you wanted to get it to work, it would take dedicated effort on both side practicing double possessions to find out how, and then more learning to find out how to stay out of your old body.
As I see it, this example given in the last thread was not a soul switch, but a possession of ONE by the other which is more like an insertion of consciousness so that one person controlled both bodies.
About walk ins:
What can happen, is the spirit chooses to leave first. This usually happens after a traumatic event like an accident that puts them near or through death, as leaving the vessel is basically dying. If the vessel is still in good condition though then another spirit can take it and this is called a “walk in”. Sometimes walk ins remember they are walk ins and have the memories of the host and themselves. Drunvalo Melchizedeck is an example of someone who talks about being a walk in as well as the lives he had before walking in to the current vessel.