Microsoft 365 features that help users manage their subscriptions, account settings, and billing information.
When you upgrade your MIcrosoft 365 subscription from the Personal version to the Family version, you would be charged the full price for the new Family subscription, and if you did not cancel the Personal subscription first, then the remaining days on the Personal Subscription are converted on a pro-rata percentage to the Family subscription.
The process is explained on this link:
Log into your Microsoft Account on this link and check the expiry date on your subscription, does that indicate more than 12 months remaining on the Microsoft 365 Family subscription.