Hey @mkblovesthedawg! Welcome to the Spotify Community. ^^
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.
Be sure to check out this Spotify Family (Q&A) for more details!
Let me know if that helps, thanks! ^^