I had an idea for a world where brain uploading is a thing, but I have a problem. Besides slowly replacing the brain itself with nanobots and the like, what's the best way to make absolutely sure continuity of consciousness occurs? I'm stumped.
We do not know where consciousness comes from exactly
There is no way of proving something has consciousness. Technically, I (or you, from your perspective) could believe that I am the only conscious being in the universe and the rest are just automatons working on natural principles resulting in the behaviour they exhibit. None of them have to be conscious for that.
It is believed that consciousness is an emergent property from brains. This means that you don't alter the presence of a consciousness if you slightly alter the brain (e.g. remove some neurons or synapses), but it requires the brain as a whole and its structure to exist.
Therefore you can come up with anything to say consciousness is preserved as we have no way of disproving you anyway.