How (un)usual is changing fields?
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.