As you already have Premium, your payment date will likely be a few days earlier than it had been. This is because of the difference between the price of Premium for Family and the regular Premium subscription you were on.
Here are a few things to keep in mind:
It’s not possible to purchase Premium for Family with a gift card.
You can’t combine it with any trials, or offers such as the student discount.
If you're subscribed to Spotify Premium through another company (e.g. your broadband/mobile provider), you need tocancel your subscriptionwith them to upgrade to Premium for Family.