Why 18-inch turrets would cripple a King George V refit

How would the King George V class fare if refitted with twin 18-inch turrets?

Poorly; the conversion would probably leave only four or five guns, turning the ships into specialized bombardment assets rather than effective battleships.

Twin 18-inch turrets would leave the King George V class with too few guns for an effective battleship battery. Fitting large replacement mountings in the lower forward and after positions looks more plausible than replacing the raised forward turret. That would leave four guns, or perhaps five if a single gun could occupy the raised position. A two-forward, two-aft arrangement resembles an older Royal Sovereign-style battery and would chiefly suit monitor duties in a dreadnought-era fleet. A Lion could more plausibly exchange nine 16-inch guns for six 18-inch weapons, retaining a Renown-sized salvo, though the benefit would still be limited.