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Only the fields supplied are changed. On an active or past-due subscription, next_billing_date and anchor_day are refused with 409 subscription_renewal_date_endpoint, even when they carry the value the subscription already has: the renewal date moves through POST /subscriptions/{id}/renewal-date, which bills the days between the old and new dates. On a pending subscription, a next_billing_date on or before the end of a period already invoiced is refused with 409 conflict, and so is a new next_billing_date while cover or a repeating charge starts on the current next_billing_date or on the agreement's move_in_date: move the agreement's move_in_date instead, which moves them. While a renewal date change is pending (pending_renewal_change is set), a change to billing_interval or interval_count is refused with 409 conflict until the change is cancelled, because the pending change was worked out for the current cadence. On a subscription with no licence fee (one with no agreement, which bills only its repeating charges) a change to amount_minor is refused with 409 conflict: its price is the sum of those charges. A new billing_interval or interval_count re-prices every repeating charge and every cover the subscription bills from its next invoice, each converted afresh from the price it was converted from (a charge's own price, a cover's premium) and never from its old amount per period; amount_minor moves by what the next invoice then bills. The licence fee is not re-priced: an amount_minor sent with the new period moves it by the difference from the current amount_minor, and the re-pricing lands on top. A charge with no price of its own on a subscription billed every two or more intervals, or one that can't be priced for the new period, refuses the whole change with 409 conflict, and nothing is written. An amount_minor equal to the current one is no change. With expected_amount_minor, a change is refused with 409 conflict when the subscription's amount_minor is no longer that figure.