Microsoft is moving to a paradigm where a Windows app is a Windows app regardless of if it's a Phone targeting app or not. So you'll see a lot of references to "Windows 10 apps" or "Universal Windows Apps" or "UWP apps" which is the new paradigm. This means targeting Windows 10 regardless of if it's on the phone or tablet or desktop and doesn't have specific references to phone.
If you developed a Universal App you're probably good to go. For Windows Phone 8 and Windows Phone 8.1 apps you may (or may not) need to make some changes to move toward the Universal type of app. It depends a little on how much you were using Silverlight (not that it's not supported but there are some changes so you'll just want to test your app).
This video blog (and the comments below it) might help explain the difference between Windows 8.1/8 Phone apps and the UWP. http://blogs.windows.com/buildingapps/2014/12/17/bring-your-windows-phone-silverlight-apps-to-windows-runtime-xaml-prepare-for-universal-app-development-in-windows-10/
Yes, with the caveat that there might always be some outliers that have compatibility problem. You can join the Windows Insiders program if you want to test for yourself.
Yes I have apps still under development which targets both WP8 and Universal Windows Phone/Windows 8.1.
But if you're talking about converting it to UWP. Based on my experience, I was able to port my codes from Universal 8.1 apps to Windows 10 with minimal adjustments. It's just the UI/UX that you may want to update.(e.g. Pivot to Hamburger Menu.)But when I tried to port my WP8 app, I was not able to build it successfully in the first place because of deprecated codes.
So to sum it up, when porting WP8 expect that there will be huge adjustments. But for the ones targeting 8.1 there will be minimal to no adjustments.
