Rekordbox License Key and resale

I just bought a new XDJ-RX2 as an upgrade to my DDJ-400. 

Now I intend to resell the DDJ-400. My question is this. Is there any difference between a DDJ-400 Rekordbox Licence Key and a XDJ-RX2 Rekordbox Licence Key? If it's just used to activate Rekordbox I could just sell the new key. 

If I can't do that, how do I unregister the DDJ-400 Licence Key from my account?