Does fear ever motivate somebody to change?

It depends what change you want.

Do you want to induce action or inaction?

For example, telling someone, If you don't go to the gym, you'll be fat.

That's a fear strategy, and you're trying to use a fear strategy to get someone to do something.

The reason it doesn't work so well is because of something known as the approach avoidance principle.

To motivate someone to act, it's better to highlight the reward, not the fear. Wow..