Sure you can do it, but the wad inside the hull is different for buckshot because an ounce of #1 balls takes up more volume/space than an ounce of #8 that is denser without all the inherent gaps larger balls create. In other words, the cavity for the payload which is made up of plastic petals is longer on buckshot loads since there is so much wasted space in there.
So if you decide to just open up the crimped petals and pour out the small shot and replace it with #1 Buck you might end up a pellet or three short, simply because of volume restrictions. But it'll certainly work!
Just make sure that the replaced buckshot weighs the same (or less) than the birdshot you pour out.
Hope this helps.
Dave