Preview a plan change

May create/link the billing account, but does not attach the plan. Returns full minor-unit quote and recurring next cycle. Use the same checkoutAttemptId and unchanged input with approvedQuote when changing the plan. 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.