Does anyone know if this is possible? I really want to pick up the Forza 6 LE bundle and sell my old unit but I want to make sure that I can register the new console in place of the old one.
It's not possible no, the console preview program is tied to both your account and hardware ID.
You'll need a brand new MS account for the new console in order to use the preview program.
Also I'm curious, why do you want to sell your old unit?