If you already have active subscribers to a subscription, we cannot change the fundamental parts to that subscription i.e. price or term, as this will also impact these existing subscribers.
In order to ensure your existing subscribers are not affected you will need to create a new subscription to include your desired changes. Once your new subscription is live, you can then hide your old subscription to ensure any new customers purchase your new updated subscription. Although hidden from the marketplace, your old subscription will still be visible in your fulfilments and reports to allow you to view and fulfil any live orders still linked to that.