-First start by opening up the old chuck all the way and you will see a screw down in the middle of the chuck, unless it is missing. (The screw is left-handed threads.)
-Remove the screw.
-Take a large Allen wrench. Tighten down in the old chuck (the chuck has right handed threads).
-Lay the tool down on its side.
-Take a hammer and hit the other end of the Allen wrench several times until the chuck comes off (some of them are on very tight).
-If it doesn't come off, then spray WD-40 down inside the chuck and let it sit overnight or for a few hours and then try it again.