Uploading, in the sense we’re using it, is an endosymbiosis you consent to. Biology has done this before; what is new is that it would happen inside a single lifetime, and that one of the parties would have an opinion about it.
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The hardest part of the companion route is not the computation.
It is the image of the body dying while you are still here.
One-neuron-at-a-time replacement is easy to teach because every step is small. The reader can be asked: which step killed you? The companion route is harder because the body seems to fall away all at once. The tissue step is large, and the imagination treats a large tissue step as a large self step.
That is the mistake.
The step is in the tissue, not in the inference. If the interface has already migrated and the inference has already migrated, then the body's failure is not the failure of the boundary. It is the shedding of a previous form.
We need a biological frame for that.
The best one is endosymbiosis.
Endosymbiosis is the family of events in which one living system comes to live inside another and the pair becomes a new biological individual at a higher level of organization. The ancestor case is mitochondria: once free-living organisms, now organelles inside our cells. The old boundary did not simply vanish. It was incorporated into a larger one. Individuality moved upward.
That is not a metaphor in the loose sense. It is the closest natural precedent for the kind of boundary change this movement is trying to imagine.
Uploading by companion is not leaving biology behind in disgust. It is not a ghost escaping meat. It is a boundary reorganization in which a larger system forms around the old one, becomes primary, and eventually carries what the old system used to carry.
The unusual part is timescale and consent.
Nature did this across evolutionary time, without anyone being asked. The proposed case would happen across one lifetime, and the person would be an active party to the absorption.
Uploading is an endosymbiosis you consent to.
The companion cannot be a passive recorder.