Whenever you downgrade/upgrade to another paid plan, you’ll get a prorated refund from Shopify based on the number of days left in your billing period. However, if you downgrade to the free plan, you won’t get the automatic prorated refund from Shopify.

More details on: https://help.shopify.com/en/manual/your-account/manage-billing/your-invoice/apps#app-prorating-upgrades-and-downgrades

Remember that when you’ve used all your displays before your renewal date, your pop-ups won’t be displayed on your website anymore. Learn more about how displays work or check out how to upgrade to a bigger plan.