Once a subscription exists, there is no API path that changes its billing_time between calendar and anniversary. A plan upgrade or downgrade silently keeps the original value (no error is rai

Once a subscription exists, there is no API path that changes its billing_time between calendar and anniversary. A plan upgrade or downgrade silently keeps the original value (no error is raised), and the subscription update endpoint does not treat billing_time as editable. Note that billing_time is a subscription attribute, not a plan attribute, so there is nothing to inherit from the new plan either way.

How to change billing time

Terminate the current subscription, then create a new one with the desired billing_time. Recreate with the same external_id so the new record is treated as the same logical subscription, and let the termination complete first. While the original subscription is still active, the create call routes through the upgrade or downgrade path and the new billing_time is ignored again. This resets any partial billing period accrual and may generate a termination invoice for the current period.

This article reflects guidance drawn from customer case resolutions. It is not officially supported documentation and may not apply to all situations.