What happens when I change plans?
Most of the time Shopify will prorate the plan when you change it. Meaning you will only get charged what you are supposed to based on the number of days you've been using the plan. Details below:
Changing from Monthly Plans
If you upgrade or downgrade your subscription with an app, then the app will prompt you to agree to a new recurring app charge. This is because Shopify allows each app only one recurring app charge to be enabled at a time. The existing recurring application charge will be canceled and replaced by the new charge.
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 a $5.00 plan, and then upgrade to a $15.00 plan on day 15 of the billing cycle, you'd be charged $5.00 + ($15.00 - $5.00) * (15/30) = $10.00 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.
Changing from Annual Plans
If a merchant switches to an annual plan with a higher price, then the new subscription charge is prorated based on the difference in plan price and the time remaining in the billing cycle.
If a merchant switches from an annual plan with a higher price to an annual plan with a lower price, or from an annual plan to a 30 day plan, then the charge for the new plan is deferred until the current plan's subscription cycle is complete. For example, if a merchant begins an annual subscription plan in January but switches to a 30 day plan in September, then the new 30 day plan will start when the annual plan expires in January.
https://shopify.dev/tutorials/create-an-annual-subscription#plan-changes