If I upgrade from Rhino 7 to 8, can I still use Rhino 7?

Yes. When you upgrade, you are extending your license to give you access to Rhino 8 features – you are not adding a new license. This means that you can use Rhino 6 and older versions of Rhino on the same computer. You may not use Rhino 7 on one computer while someone else uses the Rhino 8 upgrade on another.