We depend on Shopify for billing. This is an explanation of how Shopify handles upgrades and downgrades. More details can be found are here: Shopify explaination
When you upgrade your plan by moving from a less expensive charge to a more expensive charge, the charge is prorated based on the difference in price and the number of days remaining in the billing cycle. For example, if you begin a 30-day billing cycle on the Basic plan at $9.95, and then upgrade to Plus at $19.95 on day 15 of the billing cycle, you’d be charged $9.95 + ($19.95 - $9.95) * (15/30) = $14.95 USD.
If you downgrade your plan by moving from a more expensive charge to a less expensive charge, then you’re automatically offered an application credit based on the difference in price and the number of days remaining in the billing cycle. This application credit can be used towards any future application purchase on Shopify.