Be extremely cautious if a third-party website claims they can "convert" your iTunes card to Spotify credit. These are often scams designed to steal your gift card code. Only use your gift cards on the official Apple App Store. If you'd like to move forward, I can help you: Find the on official Spotify gift cards.
If you want to upgrade your account, you must use one of Spotify's accepted payment methods: can you buy spotify with itunes gift card
Even though both services are available on iPhones and iPads, they are direct competitors owned by different companies. Funds from an Apple gift card stay within the Apple ecosystem, while Spotify requires payment through its own supported channels. Why Apple Gift Cards Don't Work for Spotify Apple Music and Spotify are rivals. Be extremely cautious if a third-party website claims