Re: Robotc and the default Lego programming.
In theory, Ernest3.14 is correct. Although I wouldn't describe the transfer of firmware as easy; you run the risk of having to reset the NXT and you will remove all of the previous Firmware's files including the programs.
To me, it would be better (in my opinion) if your budget allows, to get another NXT and have one have the NXT and the other have the RobotC Firmwares.
Have a good day,
P.S. Have a good time programming!