New Alpha endpoints to change a subscription's price

Alpha — get in touch if you'd like access. Shapes may still change before GA.

You can now change the variant/price on an active subscription line item via the public API, using a three-step Discover → Preview → Commit flow:

• List change options — returns allowed upgrade/downgrade targets, grouped by variant, with prices inlined.
• Preview change — non-destructive quote of the upcoming invoice with previous_line_item and new_line_item blocks to diff.
• Perform change — applies the change. Same response envelope as preview, just with preview: false.

Optional prorate and effective_time (now / next_renewal) overrides are available. Defaults follow convention: upgrades prorate immediately, downgrades take effect at next renewal.