I have read many articles about the "forward compatibility" of WP apps stating an app targeting 8.0 can run on an 8.1 phone. But I have not verified this on WP Store.
I have published a universal app for WP 8.1 and some users have installed this app. Now I am investigating the various upgrading scenarios happen when Windows 10 Mobile comes out later this year.
Scenario 1: Can a user download and install the 8.1 app with a newly purchased Windows 10 phone? Will it show up in the new, unified Windows Store?
Scenario 2: Can a user who has already installed this app continue to receive updates to this app after he upgrade his phone to Windows 10 Mobile? I have no plan for a Win 10 release at this stage and I will update the 8.1 release in the months to come.
It will be helpful if you can provide me some links about the policies of the new, unified Windows Store.
Yes to both. Existing apps will show up and can be installed from the store. As with the existing stores, users will get the most relevant app for their device. Your existing Windows Phone 8.1 apps will run on and be available for Windows 10 mobile devices. You can update it and expect your users to receive the updates.
This is discussed in the store sessions from Build: All That is New in the Windows Store and Store: Deep Dive on Publishing Universal Windows Apps
The blog entry Get ready for the Unified Dev Center dashboard preview and upcoming Store changes goes into detail on what you need to do to support various scenarios with the updated store.