It's not possible to just pay the difference, but the good thing is that your remaining Premium time will be converted into Premium for Family time when upgrading.
Keep in mind though that your next payment date will likely be earlier than it had been, due to the difference between the price of Premium for Family and regular Premium. Just head here to get started.
MaximRock Star 24
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Yes, I finally purchased the family plan. There was then an automatic payment of $0.01 (1 cent) from my credit card, and a notification that my next payment was to be in 8 month (I had a year of premium already paid for, and that = 8 months of a family subscription).
The problem is, there was ZERO notifications as I was going through the front end billing pages that explained any of this. The only information given to me, until I agreed to pay, is that I would be charged $14.99/month. It was a leap of faith to press that finalization of the payment and not be double billed for my subscriptions.
Reading through the questions and comments, there are numerous other people with the same confusion who won't find the answer until the try to purchas a second subscription on top of there own. Many people have obviously hadn't (or won't do this) bacause they are worried about creating more confusion and loosing money.
The web design need to reflect the reality that only one subscription can be attached to each email or Facebook account. And whatever has been aldresdy been paid down towards a subscription will then be automatically forwarded to the next subscription on the event of a change.
It would be awesome for this to made transparent during the billing process.