Changing Billing Information
If I purchased a subscription on one card, but I changed billing information while the purchase is still pending, where will it be charged?
iPhone 12, iOS 16
Question marked as
Good question but not one I recall seeing mentioned in Apple's documentation, so ask Apple:
Choose your country or region - Official Apple Support
If I had to guess, I would think whichever payment method is in action the time the purchase was finalized.