What happens if a professor's interests shift dramatically? Let's say Alice is a professor in underwater basket weaving. She likes the topic, has several PhD students &amp; 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?
Assume that the change is gradual, not abrupt. Alice will want to learn something about the new field so that she is able to do effective research in it. No new department is likely to accept her until this happens. She is also unlikely to completely abandon the old field due to the many constraints, both financial and ethical. Yes, she needs to see to the needs of her current students somehow, and may need to keep publishing in the old field if she wants good annual evaluations. 

Once she has a few nice publications in the new area she can start to look for a new position. She probably doesn't need to get any additional degrees unless that is needed to obtain the expertise in the new field, but it would probably help her job search to have some formal qualification in the new field. 

One way to get some expertise is to try to collaborate with experts in the new field, first as a junior partner. It is best, of course, if she has something to offer her collaborators, perhaps some research perspective from the old field. 

For smaller jumps, of course, it is much easier. Many people switch back (and forth) between math and computer science, for example. This is true even though the basic research methodologies in those two are quite different. I switched from math to CS over a few years at a time when the institution I was at needed expertise in a developing field (long ago). But a person who switches here likely is able to use many of the ideas in the "old" field to enhance their work in the new.