Fist of all I want to say that I've read these two discussions: How to organize Windows Phone code base to target both 7.x and 8 platforms and Windows Phone 7/WPF - Sharing a codebase. So, I know about Portable Class Librariy project type which might be an answer to my question. But it isn't an answer for my case.
Problem
I have some old code-base in my project WindowsClassLibraryNetFw35
. I need this project to be a class library which targets .NET Framework 3.5. The reason for it is that some dependent applications are deployed on customers' machines which don't have .NET Framework 4+ installed. Unfortunately I can't force my customers to upgrade their environment.
Now I create new project(s) for Windows 8 and Windows Phone 8. As you know, Windows Phone 8 projects will fail when I attempt to reference my old library because it is not a compatible Windows Phone project (see screenshot).
Question
What is the best way to reorganize my project type(s) in order to be able to support both my old and new projects?