awesome, so you know where I can find a guide on how to do that?
Tip the keg onto it's side. Use a screwdriver to push down the silver ball valve in the center of the keg. Let the pressure off. Beer can shoot several feet, so point it where you don't mind STINKY old beer spraying.
Take a 12" piece of string. Attach a pencil to one end. Hold the other end onto the silver ball valve in the center of the keg. Pulling the string tight, and draw a 12" circle around the center of the keg.
Take a drill, and drill a small hole INSIDE the circumference of the circle (this is a double check that all the pressure is released).
Take a small (4" or so) angle grinder with a cut off blade and cut out the top, following the pencil line. Remove the top.
Take a file, sandpaper, emory cloth and/or wire brush and smooth out the edge of the cut (unless you really like stitches).
Clean that nasty thing out, and you are ready to cook. You can add a sight glass, temperature probe(s), valves, screens, filters or whatever you want at this point.
Search around the site, and you can find pics or even videos of the process.