Your project is only "changed" to VS2012 when you first open it in VS2012 (and it will prompt you before running the conversion). If you open the old project in VS2008 it will work fine.
Possibly what you're seeing is that the file association for your solution files is changed, so that they open by default in VS2012. The way around that is to open VS2008 manually and use "Open Solution" to load your old project, instead of opening the project from a direct shortcut.