Unfortunately, it is not possible to change a Subscription from one Billing Period to another.
In this scenario, you will need to cancel the Subscription, set up a new one, and move the allocation over. This is a straightforward process, and your customer doesn't need to be notified if you decide not to use it.
How to move a customer to a new Subscription
Set up a new subscription using the Create Subscription form. If you already have a payment method on file, then it will appear here, and you can charge it without speaking to the customer again.
Cancel the existing Subscription.
Deallocate the unit(s) from the cancelled Subscription
Reallocate the customer to the new Subscription and sync if using a Smart Entry system. The process will be seamless for customers, with nothing changing.
That's all there is to it!