Returns a non-destructive preview of changing the variant and price of a subscription line item, including proration math, the next charge amount, and the effective date. Follows the same upgrade/downgrade rules as the in-app self-serve flow.

Source allowed products_price_id targets from GET /products/prices/{price_id}/change_options. After previewing, call POST /orders/line_items/{line_item_id}/changes/perform with the same body to commit. The response envelope is identical between preview and commit and only the preview flag flips.

On commit, 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.