Execute a plan change (v2, swap in place)

Change a subscription's plan in place. Subscription id, billing anchor and period bounds are preserved; line items are sliced and settled in one transaction.

change_at controls timing. Omitted or 'immediate' applies the change now. 'end_of_period' records a pending schedule that executes at the subscription's current period end: the response returns is_scheduled, schedule_id and scheduled_at instead of a completed change, and nothing is swapped or billed until the boundary.

scheduled_at is resolved from the subscription's current period end at request time. If that period end is already in the past (a backdated start date, a resumed pause, or worker downtime can all leave a subscription behind), the change is due immediately and fires on the next billing scan rather than a period away — inspect scheduled_at to see this.

Only one plan change may be pending per subscription; request a second one and this returns 400. Cancel the existing schedule via POST /subscriptions/schedules/{schedule_id}/cancel first. Pending schedules are listable via GET /subscriptions/{id}/schedules.