To change the payment method from Docomo payment :
Cancel your subscription first. After your account status become "Canceled," restart your account with the payment method you want to use.
*You cannot change it to iTunes Store Payment. iTunes Store payment can be selected when you sign up from iPhone/iPad/iPod touch/Apple TV or restarting your account signed up with iTunes Store payment.
*There may be some days left until the service stops depending on your billing cycle. In this case, please restart your account after the service stops.