People change careers every day. It's not uncommon for professionals to have as many as five or more careers in a lifetime.
Entrepreneurs change careers by starting new and different businesses.
Technical people may move from design and programming into management, marketing, sales and back again. At some point you might even want to change industries. People approaching retirement may wish to start on a career that will allow them to work beyond forced retirement age. Some of these changes, such as the normal career progression from an individual contributor to a manager are easy for you to navigate. Other changes such as moving from one industry to another will be more challenging.