When you get a brushless speed controller it wil replace the speed controller you already have. Motors have a watt (or is it amps?)rating that tells you how many watts it will try to draw in order to perform at its max potential. The speed controller needs to be able to handle providing those watts from the battery. If your motor draws a max of 100 watts, your speed controller has to be rated at a minimum of 100 watts.....given your battery can provide that kind of "wattage".

Im no expert by any means, but that is what I understand. Im sure someone else can explain further.