Well there's cases of people who live normal lives even though they're missing massive portions of their brains right, I would say that we can't guarantee they are the same person but I think it'll vary from person to person but there's a certain point where changes in personality or rationality happen and I think that's when it happens, your cells always die so one doesn't mean you're an entirely different person especially if another neuron already carried the info that one had before it died