Even if it is suggested by experts to never change the tub spout when the water supply line is working, still you can do it. The process won’t be different just you don’t turn off the water. Just be sure to turn on faucets including the toilets, bathtubs, showers, basins, and so on.   This way you can safely change the tub spout by lessening the water pressure coming from the faucets. And, it will be easier to do that.