Upgrade API improvements

We’ve improved the Upgrade APIs to support more upgrade scenarios and greater control over product selection.

Previously: Multi-line subscriptions were not eligible for upgrades using APIs, even when a line item was eligible for a SKU upgrade.
Now: Multi-line subscriptions are eligible for SKU upgrades when at least one line item has an eligible SKU upgrade option. SKU upgrades only change the product without changing the contract’s frequency.

Specific product selection: When an upgrade profile has multiple products configured, merchants can now specify which product to upgrade to using the new variantShopifyId parameter. Use sourceLineId to specify the line item being upgraded.
Product quantity + frequency change upgrade: Added support for this upgrade type on single-line subscriptions using:
- isMultiProductQuantityUpgrade : Indicates whether the line should be upgraded to multiple products.
- updatedProductInfo : Specifies the variantShopifyId and quantity for each product.

Note: This is available in v2026.04 only (not v2023.10) for both Admin and Storefront APIs: