Hi John Janoudi,
Thanks for reaching out to Microsoft Q&A Forum.
About your question regarding the upgrade from Office 365 Personal to Family, when you upgrade, Microsoft doesn’t charge just the difference. Instead, the full amount for the new plan is billed, and a portion of your remaining time from the previous subscription is automatically converted and added to your new plan.
To understand how much time carries over, please refer to the chart in the attached image.
For more details, you can find in this link: https://support.microsoft.com/en-us/office/switch-between-microsoft-365-subscriptions-3fcc1efc-2722-427f-8efa-db94b9b0a36b
Besides, I recommend checking your Microsoft account to view your subscription details and confirm the adjustment.
I hope this helps you get the thing sorted out quickly!
Best regards,