I have a standalone version of LR5 currently on my Mac.
In a couple of weeks I'll be getting myself the new Mac Book Pro 15 inch.
I also want to upgrade to the standalone version of LR6
What is the best way to go about that? Should I first re-install LR5 on my new Mac? Or can I dowload the standalone LR6 version?
Keep in mind: If you update your catalog with LR6 then you cannot use it anymore with LR5.
I think the easiest is: Work with the current LR5 on your current Mac and (directly) install LR6 on your new Mac only. Copy over the LR5 catalog and open it with LR6 and it will be upgraded for the use with LR6.
However, you can choose a different approach as well.
No need to re-install Lr 5 on new machine. You can directly install Lr 6. You might need the serial key for Lr 5 and Lr 6 both while installing Lr 6.