Removing Ork weapons (any weapon actually) is a pretty easy process.
Ok, so you have your Ork fella:
Before you assemble, you'll wanna take the arm thats holding the shoota and do some amputation, do this by taking an exacto knife and slicing off most of the front of the gun.
Next, you'll want to do some clipping. If you think you may clip a little extra off, use an exacto knife and slice.
Finally, you'll want to sand the top of the hand down, so that your new weapon can rest nicely on the hand. You'll prolly have to clip off the handle
fo the new weapon for it to fit on, if they included it. If all works out well, the trigger finger, as well as the guard, will line up perfectly with the new weapon.
Hopefully this helps!