What does it mean that designed agents replace default agents?

Every deliberate agent you create replaces an unconscious default.

Example: You notice you check your phone within 30 seconds of sitting down at your desk every morning. That's a default agent: trigger (sit down), condition (phone is present), action (open email or social media). You design a replacement: same trigger (sit down), same condition (phone is present), new action (open your task list and select today's first deep-work block). The slot was never empty. You're overwriting, not installing from scratch.

Try this: Identify one recurring behavior you'd like to change. Write down its trigger, condition, and action — that's your default agent. Now design a replacement agent that uses the same trigger and condition but specifies a different action. Run the replacement for one week. Track whether the new action fires, the old action fires, or neither fires. You're not trying to be perfect — you're gathering data on whether your designed agent can occupy the slot the default agent held.