If you are going to change the barrel anyway you only real option would be to replace the tube (assuming the barrel has the lug positioned for a the higher capacity tube).
It should just unscrew but it may be in there tight or have some mile thread locker on it. You can use a strap wrench or even pliers if you wrap them in tape or something to prevent scratching of the tube. Whatever you use just be darn sure you do not dent to bend the tube if you ever forsee putting it back on the gun. You may need to apply a little heat to get it out as well, not much, you don't want to burn the finish, just enough to heat it. I would only try this if you could not remove it with other means.
The new tube can just be put in finger tight, it won't come out.