I don't think you could start the motors If was not registered. On the Dji app, before you pick your phantom, nothing needs to be powered up or connected. Top left object arrow looking thing, will show your flights and you can save them to the DJI cloud, if your flight is there you have to be registered.
Only on an iOS device though. Android users are screwed once again.
I received a used Phantom from DJI support when I sent in my P3S for repair (cracked housing). That Phantom was already registered and i can´t change the name in the DJI GO app on my Android device, the option just isn´t there.