Cannot open VS 2013 Project Files

In the past our primary mechanism for migrating C# projects from Visual Studio to Xamarin Studio was to copy the project file, rename it with a ".Mono" addition, then open it. This has served us well until 2013. When I attempt to open a 2013 project I get an error that says the project file cannot be opened and this is the top of the error details:

System.InvalidOperationException: Unknown file format: E:\d\source\SolutionFamily\Engine\Adapters\AlarmAdapter\AlarmAdapter.Mono.csproj
 at MonoDevelop.Projects.ProjectService.ReadFile(IProgressMonitor monitor, String file, Type expectedType, FileFormat& format)
 at MonoDevelop.Projects.ProjectService.<>c__DisplayClass2.<ReadSolutionItem>b__0(IProgressMonitor param0, String param1)

If I open the project file with a text editor and change the root node from this:

<Project ToolsVersion="12.0" DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">

to this:

<Project ToolsVersion="4.0" DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">

Then it loads just fine.