Allow plan change on on_hold subscriptions to recover failed first charges

When a subscription's first charge fails, it moves to on_hold. Change-plan then returns INACTIVE_SUBSCRIPTION_PLAN_CHANGE_NOT_SUPPORTED for every proration mode, because change-plan requires the subscription to be active.

The only documented route out of on_hold is the Update Payment Method API, which raises a charge for the outstanding dues at the original plan price. There is no way to drop the failed charge and bill a cheaper plan instead.

Why this matters

Most failed first charges on higher-priced plans are insufficient funds. Those customers would often happily pay for a lower tier, but today the merchant's only option is to cancel the subscription and send the customer through a fresh checkout, which loses most of them.

Proposed

Allow a downgrade on an on_hold subscription that voids the failed charge and its pending retries, and bills the new plan's price immediately.

Requested by PxlSafe (bus_0Nh5gxUDgC2DULBKTU5qH), 25 Oct 2026.