How (un)usual is changing fields? 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)
Most successful career researchers choose their field fairly early on in their career. This is often due to their approach to research; they have a fundamental question they're attempting to answer, and they spend years attacking different parts of the question through various projects. Pursuing research in this manner allows for flexibility in the research and can lead to some very interesting breakthroughs.

From what I've seen, those who do change areas of interest often do so within a single field, so that the majority of their expertise is still relevant. Yes, data analysis skills can transfer, but much of a researcher's expertise is in the form of a deep understanding of what the past and current state of the field; knowing important (and less important) papers, what different lab groups are researching, what's been tried and what hasn't. Since such a knowledge base can take many years to gain, people are often reluctant to switch.

I would venture that it's more common for a field to shift than for a single researcher to shift; i.e., some new technology or methodology allows a whole group of researchers in a given field to investigate something new or different.