This unified system is primarily available in the US and select other regions. If you are in a country where iTunes and Apple Store cards are still sold separately, you generally cannot transfer funds between the two systems.
If you have an old iTunes or App Store & iTunes Gift Card, you do not need to "buy" a different card to use it at the Apple Store. Instead, you can follow these steps:
In countries where the universal Apple Gift Card is available (like the United States), you can use this account balance to pay for products on Apple.com or within the Apple Store app . Critical Limitations
Once redeemed, the funds are added to your Apple Account Balance .