If you have an app in the Windows Store that you've been offering for free, and later convert it to be a paid app, the question is what happens with those customers who acquired it when it was free?
The answer is that app licenses are granted as either full or trial; licenses don't differentiate paid or free. So once a user has acquired a full license for the app at whatever price, they have that license permanently. Therefore a change in price only affects new customers–existing customers won't suddenly be asked to pony up some cash for continued use, which is as it should be, and they can continue to reinstall the app freely.
The bottom line is that existing customers keep their existing licenses across app updates. So if a customer has a trial of your v1 app and you make an update to v2 that no longer has a trial, the v1 app will continue to work until the trial expires.
If you have a non-expiring trial or the trial period is not yet expired, a customer can install your v2 under that license. In this case, when you make such a change in your app, be sure that v2 checks for a trial license state and prompts the user for an upgrade and/or disable v2 functionality. You cannot force them to pay, however.
In short, unchecking the trial license in the Store dashboard for your app prevents new trial licenses from being granted, but exiting ones will still work for new app updates.