By looking at the .csproj file itself, one can tell what sort of project type(s) VS is trying to open. Within the .csproj file, the project types are given as GUIDS, but a Google search should reveal what project types they represent. In my case, one of the GUIDs represented a Sharepoint project type, which is part of the Visual Studio 2005|2008 Extensions for Sharepoint. So if the extensions are not installed on the dev machine, one will see that error.

Normally, the fix would be to simply install whatever version of the extensions one needs. But I have an additional problem – the extensions are not supported on 64-bit OS’s, which is what I’m using, both on the Sharepoint (2010) servers, and on my dev machine [I’m on 64-bit Windows 7].

So, in order to even OPEN the projects, never mind upgrade them, I need to have a 32-bit development machine. Not a show-stopper, as I have a Vista VM that I can use for this purpose.

What’s [not really] funny is that one would never be able to DEBUG any of these projects within SP 2010, because obviously 2010 requires a 64-bit OS – and you can’t even open the projects on a 64-bit machine.