Once your ad spend changes you might want to change your current plan, the system does not adjust it automatically.
You can upgrade your account any time following the steps below:
Open the Upgrade page
Move the slider to pick the plan that suits you best and click "Subscribe" button
Likewise you can downgrade your subscription to a lower plan.
Make sure to read this article before you upgrade to learn how you're billed in cases where you change the plan during the billing cycle.