RE: Do I have to change my speed controller?
You do not have to change the controller, but you should be aware that if your speed controller is not programmable to cut off the motor power at 3 volts per Li Poly cell, you run the risk of ruining the battery or worse, having a fire.
If you go to a brushless motor, you would have to change it.
They make brushed motor speed controllers that are programmable pretty cheap and I would recommend you get one.
Error on the side of safety.