Change a billing plan

May charge the default method. Rechecks the complete approvedQuote before attach; changed pricing returns 409 with the current quote. Same UUID and same plan/promotion preserve existing provider and durable-checkout idempotency. Payment setup/confirmation returns requires_action and a Dashboard URL without client secrets. Existing grandfathered/Enterprise/Consent/promotion rules apply. Requires billing.write, a verified person and owner role. Subject to rollout-agent-access-writes (default off) and allow-public-agent-writes before account/provider effects. Return and person handoff URLs always use the canonical Dashboard origin; redirect URLs, payment details and provider tokens are not accepted.