When you rethink, you consider something again, often reaching the opposite conclusion. You'll rethink your decision to go bungee jumping as soon as you're standing at the top of a tall bridge getting ready to jump.
You'll have to rethink your usual two A.M. bedtime after you get a new job making donuts at five every morning. A kindergarten teacher might also need to rethink his laid-back teaching style when he's faced with an especially high-energy class. You can also use rethink as a noun: "I need to have a quick rethink about what I'll have for lunch, since the cafeteria is out of French fries."