I found it easiest to simply remove the entire head assembly and then swap out the powder slide by removing the spring. Certainly easier then removing the cam every from the base every time. Once off you can pour out the powder and cycle the bar a few times to clear it before swapping out.
Unless you have a powder cutoff, or the tube is completely empty, doing it on the machine will result in some unnecessary powder spillage, in around and under the shell plate, etc.