Well, except we forget that the _normal_ scenario for living systems is to constantly be replacing everything. Over the course of your life, my understanding is

Well, except we forget that the _normal_ scenario for living systems is to constantly be replacing everything. Over the course of your life, my understanding is some cells are replaced every few days, and some live many years, but if you live a long life, very few will be intact from your early adulthood to your twilight years. I think the distinguishing factor that people want to latch onto is that if "identity" applies to recognizable configurations or organization of parts, is replacing large chunks qualitatively different from replacing small ones (e.g. blood donation)? Maybe another interesting comparison is virus recombination or reassortment -- where meaningfully large, recognizable chunks are put together in a new arrangement. We recognize this process as giving rise to a new strain, but you could just as easily view it as a successor to one strain, but with modifications. Or changes among institutions or groups: we think the United States is a country with a ~245 year history, though its geography and population changed a lot, with sudden expansions.