The Furutaka class had their guns and turret layout completely changed (6 single 20 cm turrets -> 3 dual 20.3 cm turrets).Interestingly, I actually find this somewhat backwards - several ships actually underwent the former refits (most notably the Kongo class), while so far as I know, outside of the treaty-skirting of the Mogami-class, no warships had their main battery altered after the ship was completed, even though it should theoretically be simpler than changing the armor or engines. Even where plans existed, such as with the Scharnhorst class (and I've heard proposals existed to replace the old 16" Mark 1 guns on the Colorado class with the newer, superheavy shell capable Mark 6 guns), they were never carried out, and you'd think there would be more obstacles to doing conversions that just didn't really happen than ones that actually did.
IIRC turrets are the heaviest parts of a ship only after the engines, and because they are high up, they affect the stability of the ship more than the engines, so swapping existing guns for heavier ones are difficult unless the ship is built withstand extra weight. Even in the case of the Mogamis, the triple 155 mm turrets were replaced by dual 203 mm turrets, reducing the total number of guns.
Additionally, a lot of battleships had their guns' maximum firing angles increased during the interwar years, so maybe that was considered enough?
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