Returns the allowed upgrade and downgrade target variants for a price, grouped by target variant. Each group lists the prices on the target variant that a subscription using this source price may switch to. Use this to render dynamic upgrade pricing pages, or to source target products_price_id values for the line item change endpoints: POST /orders/line_items/{line_item_id}/changes (preview) and POST /orders/line_items/{line_item_id}/changes/perform (commit).