A twelve-gun King George V after treaty limits lapse

Could later King George V-class ships replace the twin turret with a quadruple mount?

Yes in theory, but the hull would need redesigning to support the twelve-gun arrangement.

The existing hull was designed around the twin B turret, so the change is not a simple turret swap. A revised hull could carry three quadruple 14-inch mounts once the 35,000-ton constraint disappeared. Twelve 14-inch guns would provide formidable firepower, but only after a redesign that respected the ship's internal structure. Treaty limits shaped that structure long before the ship entered service.