@Dyoung @BartoszMilewski

The important part in the original post is that we're giving the two helium atom a choice of two _same for both_ states. If one of them is here~ish and the other is 1km~ish away, they are nearly perfectly distinguishable (which does not mean that they aren't identical).

Coming back to "atoms in the body are replaced", this is most obviously effectful for carbon: C14 is unstable, so as long as that replacement is going on you have around the same ratio of C12 to C14 as the environment around you. Once you die, as long as you don't decompose (and thus stop exchanging carbon with the environment), the amount of C14 will go down as it decays, which is what makes carbon dating possible.