Could Hood's belt and deck armor have been replaced with much heavier protection?
Short answerTechnically yes, but that degree of structural reconstruction would cost much like an entirely new ship.
NotesArmor is integrated into a capital ship's structure, stability, and internal arrangement. The limiting question is therefore not physical possibility but whether the state will fund a near-total rebuild. Replacing belt and deck systems on the proposed scale would require opening and reorganizing much of the hull. The answer distinguishes a technically feasible reconstruction from an economically rational modernization.