Read-only preview of a change of the composition before committing: the steps it runs as (LinkedIn prorated now both ways; an added inbox package prorated now, a removed one leaving at the period's end; credits raised now against a top-up charge, lowered from the next period), what is due now, the next payment and the pool after. Each part named is the whole new value; an omitted part stays. No state change, no charge: show it before change_billing_subscription.

Contract:

• MCP tool preview_billing_subscription_change, registry package mcp.id/billing_subscriptions, mount id.billing.
• Operation action, response envelope action.
• Flags: read only.