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)
I think that a lot of this depends on what you define by "change fields". In my observation, there is generally a lot of commonality even in a change, when the change is successful.
For example, somebody might have a mathematical toolkit of skills that they are very good at, and discover that it is useful in an application area that they didn't originate in. As they work in that application area, it exposes them to new problems that they find they need to solve to make progress, which leads to developing new skills and interests. From those one might move again, and so on.
The researchers whose work I respect most have often been through several of these types of transitions over the course of their careers. There are always uniting themes, but the topics, techniques, and intersecting communities may well change over time.