I don't see why you can't change, we just went straight from one to the other. I didn't know if you were supposed to do it gradually or anything, but I couldn't see how you would do that, and besides he was in pain anyway so it could have only improved matters as far as I could see. If you don't want to change you could try giving some cooled boiled water to help things along (we did that too and it was surprising how much even just an oz of water helped).