Why longer-ranged secondary shells do not replace Dreadnought's battery

Would early base-bleed shells allowing 9.2-inch and 12-inch guns to share range tables change HMS Dreadnought?

Not fundamentally; longer-ranged 9.2-inch fire helps older ships, but it does not replace the weight and damage of Dreadnought's all-big-gun salvo.

The proposed ammunition would let different calibres use similar range tables and improve the effectiveness of pre-dreadnought mixed batteries. Matching ballistic performance does not make a 9.2-inch shell deliver the same impact as a 12-inch shell. Dreadnought's uniform heavy salvo aims to inflict the greatest damage possible at long range. That allows the ship to damage an opponent before closing to the distances preferred by older battleships. Better secondary-gun reach therefore leaves the main argument for an all-big-gun battery intact.