This article applies to Classic Commerce. (Looking for Contextual Commerce documentation?)
If you increase or decrease the price of a subscription product all future purchases will be billed at the new price. All existing subscriptions that customers purchased before the price change will continue to be billed at the price of the subscription on the date purchased.
Alternatively, you might want to create a new subscription product and have the current customers upgrade or downgrade their subscription. For more information, see our support article Upgrading / Downgrading / Changing Plans.