For a long time I thought that when I start my PhD, my field of research is fixed for the rest of my academic career. However, I've encountered multiple people who have a PhD in space science or particle physics, and have later moved to atmospheric science or remote sensing — one of them even after already having tenure, starting an entirely new topic while remaining at the same institute. How common or uncommon is it to change to an unrelated field within natural science? How much chance would one have of a post-doc if the newly graduated PhD has no experience with the actual topic whatsoever? Is it expected that a Postdoc spends the first 3–6 months getting into the field, or that he/she can dive into the depth right away? Of course, a lot of scientifically relevant skills (data analysis, critical thinking, programming, statistics, etc.) are in common between different fields, but how important is the content of ones experience, really?
Related but different question: How might changing topic affect a career in academia? (different because from e.g. particle physics to climate science is more than just changing topic)