[quote name='soonersfan60']Wait, the Mario Kart is a digital download code? That's awesome, because my son wants the red unit and I want the blue one, and was trying to figure out how to transfer the game to him. Can anyone confirm this? Can I just have him download Mario Kart so it's tied to his red machine?[/QUOTE]
I'm pretty sure that Mario Kart is pre-installed on the system. So, as far as I know, there's no way to transfer it to a different system.
This might have changed, but I had a DSiXL with two Brain Age games pre-installed, and neither of them transferred when I did the DSi to 3DS transfer. So, unless Nintendo has changed the policy for 3DS to 3DS transfer, it's entirely possible that the pre-installed games are tied to that specific system forever.
Again, I may be wrong, but I'd definitely look into it before assuming anything about transferring.