When you purchase a subscription through the App Store/Play Store, Apple/Google handles all the payment transactions and any changes in subscription prices. If a plan you're currently subscribed to changes its price, the new price will not take effect immediately.
Apple/Google will notify you before your next renewal date if there's a price increase. This notification will ask you to agree to the new price, and if you don't agree to this change, your subscription will be canceled automatically and not renewed after it expires.
If the price is decreased, you will continue to pay the original price you agreed to until you cancel or your subscription lapses, at which point, if you re-subscribe, you will pay the new, lower price.