If that is indeed the case, it's a silly idea to run a 48V motor at 12V (I think we can all agree with that). THe winding resistance would be too high at for the 12V supply to be able to supply enough current and insufficient current will flow through the motor. The motor will be VERY inefficient (if it can even build up enough power to overcome mechanical losses to start spinning), greatly underpowered, and it's stall current at 12V would be vastly lower than the stall current at 48V.
If you figured out a current draw of 350A by taking the power output of the motor at 48V, and assumed the it would require 350A@12V to supply the same amount of power, that is false because of the resistance in the motor windings. The stall current at 12V would be much much less than the stall current at 48V (and the stall current at 48V is much much less than 350A).
If anything, the switch for 12V operation doesn't even need to handle anything near the 48V stall currents.
For a given motor, more voltage increases current and produces more power and heat. Less voltage decreases current and produces less power and heat. A motor with a small voltage supply cannot draw higher currents than the same motor with a larger voltage because of winding resistance. Therefore, you cannot get the same amount of power from the motor when using a low voltage as you could when using a high voltage supply. If you attach a current source to try and drive a higher current through the motor to try and achieve this, you fry the windings due to IR^2 heat dissipation (or something like that anyways).