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).

See Step 1 - Discover what's allowed in the Subscription Changes Skill
for the response shape and how to treat an empty result.