Commits a variant/price change for a subscription line item. The change must be
configured as an allowed upgrade or downgrade option on the source variant. By default,
upgrades take effect immediately and are prorated; downgrades take effect at the next
renewal and are not prorated. Both behaviours can be overridden via prorate and
effective_time.

Source allowed products_price_id targets from GET /products/prices/{price_id}/change_options,
and preview the impact via POST /orders/line_items/{line_item_id}/changes before
committing here. The response envelope is the same as preview, with preview: false —
the amount fields and next_invoice_date carry the upstream invoice quote snapshotted
just before the commit was applied.

The previous_line_item block reflects the line item's pre-change state for
products_price, products_variant, original_product, and prorated. Other scalar
attributes (created_at, updated_at, quantity) reflect the post-commit state on the
underlying record.