Sorry, there is no such thing. The project upgrade procedure had an optional step to backup the initial version. If you lost it, it's gone. There is no a downgrade.
Moreover, use not just the аforementioned backup, but use some Revision Control System in all your development. If you don't, you are not really developing software, as it can be destroyed by every wrong step of system failure. You need to be able to retrace your steps in 100% of situations. Please see this discussion:
Revision control systems, which to choose from?[
^].
Now what to do in your situation, when it's a bit too late? Two things:
First, you can re-create the project using just the source code files. Create a brand-new project and add all existing source files. Figure out what are the referenced assemblies and add references. It will be really easy to do: you already have auto-converted (upgraded project). It might not be operational, but all project structure should be preserved. Open two versions of Visual Studio and use the upgraded one as the reference. This is really easy.
Also, look at this CodeProject article:
SolutionConverter[
^]. I used to use it or some similar project to downgrade my solutions for publications. If it does not work for versions you need, you can use source code and try to improve it to cover them.
—SA
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