The new exception management library I've been working on was originally targeted for .NET 4.6, changing to .NET 4.5.2 when I found that Azure websites don't support 4.6 yet. Regardless of 4.5 or 4.6, this meant trouble when I tried to integrate it with WebCopy - this product uses a mix of 3.5 and 4.0 targeted assemblies, meaning it couldn't actually reference the new library due the higher framework version.
Rather than creating several different project files with the same source but different configuration settings, I decided that I would modify the library to target multiple framework versions from the same source project, and this article describes how I did this.