Klavik

[modern spoke]

hi! yep, you can. The problem is you have not passed the subscription item ID.

you need to pass https://stripe.com/docs/api/invoices/upcoming#upcoming_invoice-subscription_items-id and set it to the si_xxx item ID from the existing subscription object (https://stripe.com/docs/api/subscriptions/object#subscription_object-items-data-id) so that you are previewing an update to that specific item(if you omit it, the API thinks you're adding a new item)