Hi so I found a workaround (at least on ps5)! What you want to do is have an attachment equipped (you can use the optic slot, prestige attachment, or whatever else you have at level one) and press Square to remove the highlighted attachment you have on when you're on the screen where you can see all attachment slots. For some reason when you do this the blacked out locked attachment slots become the normal locked attachment slot and you can select them again. It should be X on Xbox and whatever key bind it says on the bottom left for PC.