First off, Welcome to the site.
Now lets get on with it,
Start by pulling the butt stock off the back of the receiver extension (the tube). To do this pull down on the lever (the entire lever and not just cam'd to one side), then pull back on the butt stock with intent.
From there, you will need to use a torch and heat up the castle nut. When you do this, keep the flame moving and aim it towards the back of the receiver (away from the receiver). The loctite bond will break at 600*, so you just waiting for a sweet smell of the bond breaking, and not taking the tube to the point that it starts to color change.
Now with that out of the way, using a two or three point Castle nut wrench, have someone hold the receiver as you back the castle nut off. Granted you stated that it was penned, it will spin back with enough force. As for having someone hold the receiver verse vise'g it up, if you vise or block the receiver, you take a chance of twisting or crushing the receiver.
Now with the castle nut back off, if the tube will not spin back, reheat just the tube (again away from the receiver until you smell the loctite bond break).
Now the big notes/warnings:
When you spin the castle nut back, the end plate (the one you are swapping out) is going to come back and release the tension off the rear take-down pin detent spring. Make sure that you retain the spring, and even the detent in front of it inside the receiver. When you reinstall with the new plate, make sure to guide the spring back in to it doesn't get kinked.
When you spin the tube back, you will need to retain the buffer pin retainer spring inside the receiver. The front of the tube holds these in place, and when you reinstall the tube, make sure that you screw the tube in far enough to retain the buffer pin again (the end of the tube should almost kiss the center post of the buffer retainer when correctly indexed/installed.