An intelligent agent should update its plan when it learns new information which helps it accomplish its current goal more quickly. For instance, it may be the case that in the process of satisfying one goal the agent also satisfies one or more of its other goals. The agent should recognize when it has already satisfied a goal and change its plan accordingly.
In addition, an agent should replan when facts about the world upon which its current plan are based change. This is important when in the act of achieving one goal the agent undoes another. The agent must realize this and update its plan to satisfy both goals.
Replanning is a capability that arises from other capabilities, namely planning (duh!) and interuptability.