See the question and my original answer on StackOverflow

Any Windows app can still use Windows.Storage API (StorageFolder, StorageFile, etc.) but obviously, if it's not packaged, it won't be able to use methods related to packaging (like Windows.ApplicationModel.AppInfo, Windows.ApplicationModel.Package, ApplicationData.Current.LocalFolder, etc.).

So, from an unpackaged app that was previously packaged, you can keep most of your Windows.Storage API based code. The equivalent of


can be replaced by something like


with myOldPackageFamilyName being the value of Windows.ApplicationModel.AppInfo.Current.PackageFamilyName from the packaged version.