Albert Schweitzer is my favorite career-switch example. Unfortunately I find large parts of studying to become a doctor rather dull - I've actually taken a couple hundred hours of courses. Also, thanks to the Internet and all the information on it I would not feel as good about myself as Schweitzer did because I would be only too aware of global forces beyond anyone's control that let any individual efforts seem completely useless.

For example, the recent headlines about the outburst of violence in Ethiopia. Imagine you were one of the people who spent a decade or more building a business there to help the people - only to see it all go up in smoke in the course of an hour of rioting because larger issues in the country remained unaddressed. It feels to me like I'm waiting for larger issues to result in a "signal", and that until then individual efforts are actually pretty useless because greater forces are at work pointing into another direction. So what I do now is work on my skills and on my continued education.

By the way, speaking of studying, I started a math study (I already did CS and half of "business administration"), but during the first year I found out that while it was extremely interesting it was way, way too narrow. I had too much of a desire to try something completely different. That's why I ended up studying a lot of medical topics. I think edX and Coursera and the course websites from Stanford, MIT and many others are a very good option these days. After all, you only really need one degree to say "I studied", actually getting another one has greatly diminished returns. I'm not talking about the knowledge you gain but about the piece of paper. As for the knowledge, I would find studying anything too narrow and limiting. Picking up whatever I want from the available (university) courses has been much more satisfying. 70 courses thus far and counting :-) Added benefit: No time pressure. Learn whatever whenever and wherever.