I agree with Adeline he has to learn the respect of the hand because if he doesn't with the harder he won't feel anything with the softer bit. But if you put the softer, when he goes forward too quickly, make some circles. When'll take some speed, do circles as he won't do it anymore because he'll expect that you'll ask something else. But at least during the first times, try to don't stay on lines at all.
I think first of all before changing you should with the harder being softer with your hand to see how your horse is reacting and if he respects your new way to ride!