To whom it may concern and find this usefull in future,
I have a Fiio M6 device and was trying to use my old Poweramp license, purchased via Google Play back in 2012 - As I didnt have a purchase order ID, since this is only available for purchases made via the website (direct), and the Fiio M6 does not support google services, I contacted the Poweramp support via the email Poweramp.maxmpz@gmail.com but had no lucky on converting my old license, so had to purchase a new license via website, single device and the registration process in M6 was smooth using the Order ID and email used on the purchase - All is working fine.
It's a shame I had to rebuy the player - But I accepted it since Poweramp is infinitely better than the stock Fiio Music player.