Up grade from 6 to 9, please help/advise

Been using Cubase 6 on a MacBook Pro with OS Lion ( 10.8.5 ) which is the last OS 6 will work with… I just installed a new SSD hard drive in my Mac and put my old hard drive in an external box. How should I do the upgrade? Use my old startup drive with Lion OS or start the computer on the new drive with Sierra ( OS 10.12 ) or does it matter. The upgrade is just a download from Steinberg and Im not sure how to do it… thanks in advance for the help.

New disk and OS, no need for anything other then your dongle. But keep your old disk and cubase 6 for now as a back up.
Some mac users complain about very slow graphics with the latest OSes and latest Cubases (from 8 or 8.5).