Two issues here, I think.
One, the wax inside the hive is warm and soft, so the bees CAN rework it. Bees aren't noted for super strong jaws, if the wax is a cold, hard lump they can't do much with it.
Two, fresh wax, even wax in fairly new comb, contains quite a bit of material that evaporates with time, making the wax harder. If you can smell it, it's exuding something volatile, after all. That means that new wax and old wax are not chemically the same, and I suspect old wax will not stick together well enough when manipulated by the bees to use for cells. Capping for brood is porous, unlike honey capping, so old wax works well there, and a mix of old and new wax will work well enough for the thick walls of queen cells (also porous, I think), but for brood or honey comb, only fresh wax will work.