When are we not ourselves?

[ohreally]

But every atom in my brain is replaced every few years. I can't be the same person if my atoms aren't the same.

I am a copy of a person younger than myself.

I would say you are looking at it in the wrong way.  Exactly what day/time/age of a person are we to measure to compare atoms to?  Is there no change before that?

By your method the baby being born isn't even the same person as when the first cell division happened so how can you ever be the same as you were before.