E6 Product Key

HI guys

I have accidentally redeemed my cousin's R10's product key into my E6 portal account so I know have two E6 licences (one from my R10 and one from his)

How can I transfer the E6 license from my account to him? 

What happens if/when someone decides to sell their R10 - how would they transfer the E6 licence? 

Does Garmin issue new product keys?

Many thanks