DEPRECATED: use change-package with explicit tiers, which applies every dimension — and the platform fee every non-Mighty plan carries — in one call. Add, switch, or remove the compute (machine) tier on the org's active Pro subscription. A set value swaps the Stripe subscription item in place (or adds one when the subscription has no machine item); a null machine_tier removes the item entirely (back to the small baseline). Upgrades — including adding a tier — invoice immediately (always_invoice) and grow the org's assistants up to the new ceiling asynchronously, the same way a Base→Pro subscribe does; the response does not wait on that rollout. Downgrades and removals net onto the next invoice (create_prorations, no refund) and cap any pods above the new ceiling via vembda. Rejected with HTTP 409 while a cancellation is pending. Requires an active Pro subscription with the multi-item structure; legacy single-price subscriptions get HTTP 409 until they are migrated.