It's fairly easy.
Make sure both tanks are empty and remove a either the gauge or fill nipple and then opposing burst disc; I would recommend writing/etching very lightly what port is for what since it's critical to get the burst discs back in the proper place.
Use a strap wrench to hold the tanks and a proper sized wrench to remove the regs. Screw them back on hand tight, ensuring it's not cross threading and the o-ring is not coming out of the groove. Take the wrench and tighten it back up, ensuring it can't be removed without tool force. Place the burst discs/gauge/nipple back into it's proper place and be on you way.