I have a visual studio 2008 project if I open it with 2015 visual studio can I open it again with visual studio 2008?
No, the project file will be upgraded and cannot be opened with the previous version of Visual Studio.
However Visual Studio will create a backup of the old project file and other files in your project that will be modified on upgrade.
Using this backup is your only chance to open the project with an earlier version of Visual Studio again.