What heuhen means can be clearly visualized and understood if you take the real example from land warfare, which is the Sherman tank.
The original tank in question came with a fairly roomy space that allowed three men and an American version of the French 75/40 Mlle 1898 to operate together.
Events and circumstances compelled the British to try and fit a much larger 17 pounder/(76mm/50 gun) and its attendant larger ammunition into the Yank tank to match German new tank developments.
Now understand, that at the time existing British tanks could not take larger guns because the turret ball race, (the circle of ball bearings on which the actual tank gun house sits), was not large enough to put a larger gun and crowd three men and a useful amount of ammo into. Not even the Churchill was big enough. They tried by widening that tank but found they produced a land monstrosity that bogged (land floatation) and could not fit their railroad flatcars or tank transporters.
The Sherman had an unusually large ring and its existing gun-house was barely able to take a reduced size 17 pounder. The problems that still entailed required the British to turn the gun sideways onto the trunnion, move the coax up, train the loader to load from the wrong side take the radio out of the rear, and mount that outside, shove the TC over and cut off the rear gun-house armor and fashion an elongated and thinned plate which they slapped on to close the hole to clear the gun's rather long recoil path. The ammunition stowage arrangements were also redefined and laid out in a fashion even more awkward than the usual British practice of the time. It was an utter bodge, and the British knew it. But to get a Panther killer, they had to do it.
The Americans when they went from the 75 mm howitzer to the more powerful 76 mm gun simply took a whole new gun-house purpose designed and slapped it onto the old Sherman hull. They still had to rearrange the ammunition stowage for the new larger shell/cartridge rounds, but since they had to figure out a safer stowage for the ammunition, they would do that anyway.
The Italians (remember them?) did something like that to their battleships.
Comte de Cavour as originally built.
As she ended up.
Quite a difference.
Almost a whole new ship.
The Italians remained stuck with the barbettes that the original Cavour had. That is the hoist machinery, safety locks, shell rooms, and bag rooms and the platform ball ring race that the original dreadnought was built with on which the actual guns platform with its receiving machinery, trunnions, elevation gear, and gun barrels was mounted. There was nothing the rebuilders could do about it. It was all integral to the hull framing. (Sort of like the hull sponson stowage that is in the Sherman example.)
Very complex. The Italians could (like the Americans, British, and French did) treaty cheat and revise the elevation gear and pit the guns' barrels deeper so they could elevate the azimuths for greater range, but the Italians were more clever than that. They measured barrel safety factors and determined that they could bore the barrels out to take larger diameter shells. So they thinned the 12 inch diameter tube liners on the guns' barrels to produce 12.6 inch diameter bored out guns.
With the new larger guns came the new larger shells, with the attendant rework of the shell and bag charge rooms, the receiving machinery and hoists and so forth. You will also notice that the gun-houses were also redesigned (Remember the Sherman Firefly example above?) to account for the increased pit and recoil run required for the bored out guns.
If you think that work was extensive and complex, the work that went into re-engining the ship and working out revised floatation sectional coefficients so that the lengthened hull would not hog or swang was almost twice as complex and expensive. The armor scheme had to be redistributed to make sure the floatation was correct and the hull bulged, lengthened, and re-plated to ensure that drag coefficients and desired speed was attained. Only 40% of the original ship was left when the Italians were through.
It was Italy's battleship equivalent of the US aircraft carrier 1950 FRAM program and
it was a colossal waste of money, time and expertise.
The better use of the hulls would be hard to justify even with the hindsight advantage, but if I was putting out that kind of money and scarce human engineering capital , I would have done the Lexington, Saratoga, thing on those six obsolete hulls.
Even a slow carrier is better than no carrier. And the engineering time and the industrial effort expended is no worse, even if it is as complex. (maybe more complex, if the over engineered Aquila is an example).
As for increasing existing battleship DN/PDN power, you could keep to the KISS principle. Do what the Americans did. Solve the accuracy and throw weight problem in the ammunition, not the gun. Heavier shell, hotter propellants, revised shell ogive=longer range, less dispersion, greater accuracy in the ballistics.
Speed of loading? Get rid of the hydraulics and electrify the hoists, traverse and elevation machinery. Install stiffer short travel recoil buffers and a constant rest reload stabilizer cradle while you are it. The existing pits work with the short recoils produced.
How about accuracy and speed in the fire control? Analog computer for 2d axis shooting and better optics in the central fire director. Radar if you got it.
In other words, don't go at the problem for more than its worth.
The Sherman tank example is the illustration from the American perspective again. They had a failed experimental tank but with a successful 76 mm gun and gun-house designed for it, the T 2x series. The ball race ring on the T-20 and M-4 hulls were
identical.Side hull sponson stowage was fairly faulty but easy to revise in a Sherman for the longer and fatter shells/casings.
It was a very simple fix. Worth the little added money and time expended for the obsolete Sherman to keep it effective through the war. And they did it.