When battleship rearming becomes a new-ship problem

How difficult is it to rearm a battleship with a different caliber of main gun?

Small caliber increases may be manageable, but large jumps force changes to turrets, shell handling, balance, and protection so extensive that a new ship can be simpler.

Moving from 13.5-inch/45 to 14-inch/45 guns adds 22.5 inches to barrel length, while a move from 14-inch/45 to 15-inch/45 adds 45 inches. The larger step also brings a much greater change in shell weight and handling equipment. Modifying shell lifts and machinery can therefore become only part of the work. Replacing triple 11-inch mountings with twin 15-inch guns requires much more extensive turret changes, as the discussion of Gneisenau illustrates. Ship balance must be recalculated, and retaining the old armor leaves a ship whose protection does not match its new guns. At that scale, building a new hull around the turrets may be simpler.