ok thanks for the info. My problem is that my windows 10 PC did not have the 10240 sdk installed, but by right clicking the unloaded project (because of the missing requirements) I was able to edit the csproj file in VS. I could not get to properties since the project could not load, after my changes I could reload the project by right clicking it in VS and all was good, everything compiled.
I would believe that most users on Windows 10 devices do get the major windows 10 retail builds updates when they are released, and if someone wants to publish to XBox One UWP, would only work from 14393 (which will still work even though Xbox One have been updated to 15063 as well). The only exceptions might be those like myself who have windows phones that never got passed the anniversary update. But i understand your point here.
Now I need to learn how to use the XBox Live WinRT C# SDK to get some xbox live functionality (only will use what is compatible with Creators Program though, since I am not a ID@XBox member. Will read the microsoft developer pages about that for creators program, how to use XBox Live in VS.
One suggestion though: if you intend to make 15063 default, it might still be advisable to have a SDK version selector in Xenko for UWP developement. My experiences so far with trying to compile XBox Live code with that Creators SDK have not been a huge success (using Unity and VS 2017. Fell back to VS 2015 and Anniversary 14393, which will still work on a windows 10.0.15063 machine). FWIW, I had less trouble getting things to work out with Xenko at first use.