You can actually have two ore more versions of grasshopper installed at once. What I do is have v0.6 installed first and renamed the folder like v0.6... and then install v0.8 as normal.
If you type grasshopper in the command line then by default v0.8 loads, however, if you go to the plug-ins tab and "install" GH before it is run you can direct it to the v0.6 dll.