There is no automatic way to downgrade a solution. You can try to open it with lower version of plug-in and see if you are able to resolve errors and then you have to go through testing process. Since there are new features introduced since 8.0.1095 it may require to rework your code to be compatible with older functionality.
I was able to recompile the solution after all, I had to open the project using Notepad++ and change the name of some DLLs, in older version they needed to start with "Openspan" whereas the newer one uses "Pega". Then also had to edit the ".os" files and remove the other references to the new DLLs that were not part of the older version. With that I was able to make the project work, I only had to rework an excel connector that I used.