From your Shopify admin, you can view which subscriptions, including which app subscriptions, have recurring charges.
If you stop paying your bills, or deactivate your store, then Shopify freezes your account and your recurring app charges. If you start your payments again, or re-open your store within 30 days, Shopify unfreezes your account and your recurring app charges resume.
If you pause your store using the Pause and Build plan, you should consider uninstalling any third-party apps that have recurring charges so that you're not billed for those apps after you pause your store.
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.
If the new app charge is applied immediately, then 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 upgrade to a $15.00 plan on day 15 of the billing cycle, then you're charged $5.00 + ($15.00 - $5.00) * (15/30) = $10.00 USD.
If the new app charge is applied immediately, then when you downgrade your plan by moving from a more expensive charge to a less expensive charge, 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.