Hello,
Welcome to Microsoft Q&A!
The upgrade of UWP apps is managed by the Microsoft Store. When you submit a new package in the store, the Microsoft Store will automatically update the app after detecting the version update. The UWP app cannot prevent itself from upgrading. That means all the users will update your app to the latest version which uses subscription.
You can consider keeping the old version of the application, then create a new application, and encourage users to migrate to the new version if they are willing to pay for the subscription.
In this way, you can continue to maintain a one-time payment in the old application, use the annual subscription in the new application.
Thanks.