Upgrade or downgrade to another plan (dashboard session required)

Moves this account to another rung of the published pricing ladder and re-derives the ceiling enforced against every API key on it, so an upgrade is usable immediately rather than at the next invoice. The change is STAGED, not overwritten: the current billing window is closed at this instant and priced against the tier that was actually in force for it, and the incoming tier opens the next window with the original billing anniversary preserved. A downgrade therefore never re-prices days you already consumed on the dearer plan. Refused with 409 conflict (details.reason provider_owned_base_fee) when Square’s own recurring subscription holds your price (baseFeeOwner is provider) — read planChangesGoThrough from GET /v1/subscription first. That case cannot be moved from here at all: this API holds no Square subscription of yours to change, and POST /v1/checkout would create a SECOND one and charge you for both. Cancel with Square and buy the tier you want, or contact support. AUTHENTICATION IS YOUR DASHBOARD SESSION, NOT AN API KEY. Send the @sp/core session cookie this API sets when you sign in, from the same origin. An API key is refused here on purpose: it is a data-plane credential that lives in CI and in third-party integrations, and it must never be able to change what you are billed, cancel your account, or read your payment history. Errors: 401 unauthenticated when there is no valid session, 403 forbidden when the request is cross-origin or its x-csrf-token header is missing or wrong.