The mag tubes come out fairly easily. They're threaded into the receiver, as you've probably guessed. Care needs to be taken when you're unscrewing the tube, as the only part you can apply any force to is the end the barrel lug screws into. Here's how I did mine:
First- check that the gun is empty. Check again. Now remove the barrel, trigger group, bolt, and shell stops. Place the receiver in a padded vise. Using a pair of vise grips padded with leather or a strap wrenck, grap the magazine tube at the end where the barrel attached *not* the receiver end.
You may need to apply some gentle heat with a heat gun to loosen the tube. Turn the tube counter clockwise. and it should come out without too much trouble.
Replace your 6-shot mag with the 8-shot mag, and reassemble.
