Is it possible to get a downgrade license to 2015 PRO 64 bit from my curent 2018 Pro?

PRSS, If you are only using it for non-commercial use (even though you have a licensed Pro… I have 2013 to 2018 licenses though I’m non-commercial) there should be no legal issues in using 2015 Make. A quick search showed up this link,

Let your conscience and Karma determine your next move.

Unfortunately there is a FAT line between legality and common sense.