I assume that in addition to the renamed set of files, you also still maintain a complete "parallel" set of the original files in some other directory, am I right?
Assuming you have both versions, what I would do is:
Get a file comparison tool like Beyond Compare or DiffMerge and compare the old SLN file and the new SLN file side-by-side. Also do this for each "proj" file and any other "config" type files.
It is possible to edit these files by hand. Usually looking at what is different between two copies will help illuminate what you should do to get the second one working.
You might as well start tinkering with the renamed project by hand, anyway, given that it already isn't working. You can't make it much worse. And: you might learn some handy tricks about the XML structure of these files.
Even if you do make small mistakes when hand-tweaking this files, I have repeatedly been very impressed by how Visual Studio handles things. Visual Studio will usually tell you exactly where you got it wrong.