I paid $99 to get a yearly subscription to Spotify Premium, and it will expire in December 2018. I would now like to upgrade to the Spotify Family subscription. How can I do this if I paid the one time fee for a year?
When you upgrade your subscription from regular Premium to Premium for Family, any remaining time that you've already paid for will be converted into a Premium for Family subscription, so this can bring your next payment date forward. You can review your renewal date on the subscription page.