I heard about this in a My Little Pony fanfic: replace a single neuron with a computer-controlled, interoperable equivalent. You feel totally normal. Replace some adjacent neurons. You still feel normal. Keep replacing neurons. Only the neurons at the edge have to interact with physical neurons. The ones in the middle are fully emulated. At no point have you lost consciousness. Eventually, you'll have replaced yr entire brain & you don't feel any different.
I also heard this from a philosopher decades ago. But I recently saw it referenced in a MLP fanfic. But it's from the "Friendship Is Optimal" universe, which may have been started by a disciple of some of these transhumanist grifters. But there's some good sci fi in there.