Let's say Alice is a professor in underwater basket weaving. She likes the topic, has several PhD students & postdocs, a lab, and so on. But then one day, her husband Bob is knocked down by a car and loses both his legs. Suddenly underwater basket weaving doesn't seem so interesting anymore, and she wonders why she's working on it instead of on robotic limbs.
I imagine in situations like this must have happened before. What usually happens in these cases? I'm guessing the obvious thing to do is to finish supervising all the PhD students, wait till the postdoc contracts run out, and then switch. But a shift like this would probably mean a change in department as well. How would that work? Would the new department even be willing to accept Alice? Alternatively she could stay in her old department, but then she would be a professor of underwater basket weaving that's working on robotic limbs. That doesn't seem sensible (the old department would presumably not be happy, and even if she's tenured such that the old department can't just get rid of her, how is she going to assemble a new research group?).
Also, assuming that robotic limbs is a very different field from underwater basket weaving, Alice is likely to start as a novice. How would that work then? Does she enroll as a Bachelor's student? Does that even make sense?
What is the process of switching fields like?
Related, but considering relatively similar fields with probably different motivations for changing fields: How do academics change their area of research in the middle of their academic career?