Great Sword Sigils
You should be able to put any other weapon sigil in except another accuracy one. Equip the GS, have the new sigil in inventory, double click the sigil. You get a pop up window with the accuracy sigil in one slot and the other slot empty. Click on the other slot and/or drag the sigil to it. Another popup will ask for confirmation, at least if you’re overwriting an existing sigil. That should get it installed (if somehow it wants to overwrite the accuracy sigil, tell it No).
Oh wow how I didn’t notice that is beyond me , thanks lol.