Nothing in your panel changes when you cross a slab boundary. Same storefront, same admin, same domain — only the month-end invoice reflects the higher volume.
Plan A suits new stores finding their footing. When monthly orders pass 5,000 the invoice moves to Plan B automatically; there is no upgrade button to press and no downtime involved.
Past 15,000 orders a month you are on custom pricing territory. Open a support ticket and we will arrange a slab that fits — high-volume stores should never pay list price by accident.