Microsoft 365 features that help users manage their subscriptions, account settings, and billing information.
Upgrading from one version of Microsoft 365 like going form the Personal version to the Family version can be easily done by just adding the new subscription, however, you are switching to a version of Microsoft 365 with fewer features (Family to Personal), so that will not work.
You need to cancel the existing Microsoft 365 Family subscription and disable recurring billing on the subscription, then wait for that to expire before starting the new subscription and yes, you can purchase the new subscription code form your local retailer and add the product key to your account on this link: https://setup.office.com/Home?wa=wsignin1.0
Provided you use the exact same Microsoft account for the new subscription, you will still have access to all your files and the same 1TB of OneDrive storage on OneDrive.