If you have an active subscription through an iTunes account and want to link it to a different email address, this is possible!

You need to transfer your old account, including your subscription and library, to your new account.

On the smartphone:

1. Log out.

2. Open the app and create a new account by selecting "Email".

3. Go to "My Account" >> "Restore My Subscription".

4. Sign in on all your devices with your new email address.