I've got a Windows Phone 8.1 app already published in the Windows Phone Store. I've made the app using WinRT, to make sure it can be ported to Win10 UAP/UWP easily. Now I want to make the port, but I haven't found a way in the new Windows Dev Center to upload 2 or more packages for the same app identity. What I want to do is keep the existing app for WP8.1 phone owners, and also add a new Win10 UWP app with the same app name/identity for Win10/WP10 users. So when someone searches for my app name in the Windows Store from a Win10 device, he should get the new app; but when someone searches for the same app name on a WP8.1 device, he should also get a hit for the existing (old) WP8.1 app. I would like to avoid maintaining 2 different apps with different names and identities in the Windows Store if possible.
I believe that the new unified store meets your requirements.
For example... An app Identity 'foobar' can have several packages for each platform - WP8.1, Win8.1 and Win10 UWP. App description and screenshots are shared.
And, from WP8.1 Storefront App, the user can see, search and download/install only WP8.1 ver of the app 'foobar'.
To use this feature, you need to associate your new app with already existing app on the store. Following msdn document may help you.
Adding packages for Windows 10 to a previously-published app