Hi Shai, did you get any confirmation on if it is possible to test these scenarios?
In my UI, I provide subscription options for customers to buy, upgrade, and downgrade.
For upgrades, it's successful. The upgrade is taking immediate effect although the prior subscription being overriden's expiration date is unaltered.
For downgrades, it fails in the SKPaymentTransactionObserver with the localized error "Cannot connect to iTunes Store" everytime.
For crossgrades, I'm not sure I'll be using them so haven't tested yet.