Ok, wait. I still have Cubase 6.07, 7.5 and 8. programs running separately on the same computer. I think I misunderstood.
I thought you were asking if C8 would open previous versions files.
Yes, it is a separate program as always. Different Preferences. Shares same VST folder if you wish. The license covers all previous purchased versions.
My UAD and ILok are not setup for C6 while offline, but that is another issue...