The stanchion is secured to the stud with one screw just above the toerail - after that the stanchion just pulls out of the stud. The stud is screwed through the toerail and the deck/hull joint underneath. To get to the nut below, you'll have to remove the small strip of ply veneer from the top and rear of the lockers. The pushpit ones are actually accessible (more or less) from the lazarettes. An articulated spanner extension is helpful.
However, if you're just replacing them, you can leave the studs in place (assuming the sealant is intact and they aren't leaking) and just adapt the new arch to the existing studs. Please do show/tell us about your new arch (perhaps in a separate thread), as I want one too eventually.