There are a number of questions here about changing-fields. Those are mostly about how common it is, how it affects future career, or whether a move from X to Y is feasible (example 1, 2, 3). At any rate, it does happen — I have come across people with background in particle physics, astrophysics, or space science, who were all in postdocs in atmospheric science. A frequent motivation may be to resolve the two body problem. And some fields have a higher ratio of jobs to PhD graduates than others.
But if someone with a PhD in particle physics, astrophysics, or space science, applies for a post-doc in atmospheric science, how do they overcome the competition with candidates who do have PhDs specialising in (the right subfield of) atmospheric science? Wouldn't a potential host quickly dismiss candidates who show neither experience nor interest in the relevant field, even if they have spoken the person, when other applicants are pre-trained "locals"?