Upgrade, downgrade or change interval
Plan and billing interval are two independent settings you can change at any time.
Both live on Dashboard → Billing. Moving from monthly Pro to yearly Pro changes only the interval; moving from Pro to Business changes only the plan. Upgraded features are available as soon as the change is recorded.
Yearly billing charges once for twelve months. The saving shown on the pricing page is computed from the two prices, so it is exactly what you will pay rather than a rounded headline.