The Gold to Diamond Chest Upgrade is used to transform an already placed Gold Chest into a Diamond Chest without displacing the items inside. Right-clicking on an Gold Chest will turn it into a Diamond Chest, albeit without destroying any items inside the Gold Chest.
The Diamond Chest has 108 inventory slots, compared to the Gold Chest's 81 inventory slots.
Alternatively, the player could craft a Diamond Chest using a pre-existing Gold Chest, though it would require breaking the chest. The Gold to Diamond Chest Upgrade uses one more Gold Ingot than the Diamond Chest's recipe.