If the stock is pinned it's likely the castle nut is staked as well . See picture it's the lttle dimple/dot in the notch of the casle nut . If yours is staked you will need to grind/file that away to unscrew the nut . If you can just remove the pin by punch or by unscrewing it .that would be best , you should then be able to remove the old one and slide on the new one or even keep the same one on there . It then should be Collapsible the way it was designed
