It is currently not possible to change the price of a subscription. You can create a new, cheaper subscription plan, but you cannot change the price of an existing subscription.
Of course, you could charge if the user subscribes for a year instead of a month, but I'm not sure if this suits your needs.
Another option is to offer a โdowngradeโ in a month, which seems like your decision, but unfortunately there is no official way to do this.
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