Is there any way that I can use the latest IDE (VS2015) to work with my old solution project but i don't wish to upgrade my project?
Unfortunately, you cannot do this. The format of project and solution files was changed in VS 2010, so the furthest back you could possibly go with VS 2015 would be to VS 2010.* And that wouldn't help you any for cross-compatibility, since VS 2008 can't open VS 2010 project files.
You have no real choice but to allow the IDE to convert your solution and project files to the latest version. However, this will not delete your old VS 2008 project/solution files. They will still be in the solution directory, and will still work for opening the solution in VS 2008. The new project/solution files will open the solution in VS 2015.
This was a very common strategy employed by open-source projects before recent versions of Visual Studio implemented cross-compatibility. They would just have multiple versions of project and solution files for each supported version of Visual Studio.
The only disadvantage with this is that changes you make to one version of a project or solution file don't propagate to the others automatically. But this isn't really that big of a deal, since once a project is reasonably mature, the project and solution files don't change very often.
* In fact, VS 2015 interoperates smoothly with project/solution files from VS 2013, VS 2012, and VS 2010 SP1. It can create and modify project files for any of these versions. See MSDN: Porting, Migrating, and Upgrading Visual Studio Projects (for Visual Studio 2015)