In January '18, I prepaid a full year of Premium for myself. Now I'd like to covert to Premium Family but can't find a definite answer for what would happen to my bill. How does my $119 CAD payment get applied going forward?
Keep in mind that the payment for one year of Premium is only available for individual subscriptions.
You can still upgrade to Premium for Family here. Once you do it, you won't be charged again right away. The remaining time of regular Premium that you paid for, but haven't used yet, will convert into time for the Premium for Family plan. However keep in mind that this will make your next payment date to come forward a few weeks as the Premium for Family plan is more expensive as the regular Premium subscription.
Let us know how it goes or if you have questions. Have a great day!