Are billing cycle lengths changeable at the customer subscription level?

The billing interval for a subscription's billing cycle is stored at the plan level. This means that any plan created with an annual renewal will always be billed a year to date after the most recent billing. Even if the next renewal date is changed on the individual customer subscription, the next renewal afterward would take place a year from that date and time. You may want to create a new plan with a different billing period length and change the customer over to the new plan in order to continue billing on a different cycle.