You may call Apple at 1-800-MY-APPLE to request a balance transfer from the store card to an iTunes/App Store code.
You cannot use an existing Apple Account balance to buy or send new gift cards. buy itunes card with apple store gift card
All gift cards must be redeemed in the same country or region where they were purchased. How to Use Your Funds You may call Apple at 1-800-MY-APPLE to request
Some users have successfully exchanged or used these cards at physical Apple Store locations to facilitate a switch, though this is subject to current store policy. Important Safety Note Apple Gift Card buy itunes card with apple store gift card
If you have the older physical-only card and want to use it for digital items: