Possibly of interest. There were fairly detailed engineering studies of the Furious, Courageous, and Glorious before WWII, which included serious rebuilds. Somewhere on the internet there is (or was) a report on this. From memory, the rebuilds included:
Extending the flight deck froward and incorporating a hurricane bow.
Improved elevator arrangements.
More powerful catapults.
New bulges - for better protection and for buoyancy/stability. The new internal space forward of the hangars would be used for crew quarters and such.
Updated machinery similar to what was done to the 3x QE class and Renown.
Improved AA armament, etc
Burt's "British Battleships 1919 - 1945" contains some details. According to that it was a 1938 proposal to convert C&G only (Moore in "Building for Victory" however indicates all 3 ships were to be included), to single hangar ships. Hangar had to be at least 62ft wide (increased from 50 feet) to allow stowage 3 abreast as in the Illustrious class, height had to be sufficient to allow 3 gallery decks of reasonable width for accomodation. Beam would have to increase by 14ft.
He provides a drawing of the midships section of the proposal. So although the beam was only to increase by 14ft it seems to have involved not just bigger bulges, but a complete reshaping of the external hull with vertical sides instead of outward sloping sides to generate the extra width. So at hangar deck level a lot more space is generated than an increase in beam of 14ft by bulging might suggest. Overall the ship would have had a lower profile. The extent of that level of reconstruction seems to me to far exceed anything with any of the capital ships of any navy and that carried out by the Japanese on Kaga & Akagi in the1930s.
Those are huge changes which would have been costly both in terms of time (years out of service) and money. Moore notes that the cost of these reconstructions, including incorporation of new machinery, would be almost the same as building a new carrier of the same displacement.
By then C&G were already 20+ years old, and, IIRC, could be replaced by new construction anyway under the terms of the various Treaties. So what is the advantage of rebuilding them? And while they are out of service RN carrier numbers are reduced.
It is worthwhile remembering that, at least for the RN, warship reconstructions were not a replacement for new construction. They were solely to extend their life (capital ships) or repurpose them (AA cruiser conversions) until new construction of better, more modern ships could be built and brought into service. By the end of 1938 the carrier build programme already had 5 ships building (Ark just entering service and 4 Illustrious) and one on order (Implacable) plus agreed plans to build 1 more per year in the 1939-41 programmes. Indefatigable (the 1939 ship) was certainly seen as a replacement for one of the early carriers, Argus or Eagle if not both. Consideration was being given 1938/39 to extending the carrier build programme 1942-44 specifically to build replacements for F,C&G.
I can understand the desire for a wider hangar. At 50ft you can only fit 2 Swordfish/Albacore/Barracuda abreast. And with an increase in length from 36ft in the Swordfish to 40ft in the Albacore/Barracuda, aircraft capacity would be falling. Capacity can begin to be restored at 62ft.
By May 1939 RN plans looking forward to 1942, with the first 6 Armoured carriers in service, would have seen all 3 ships in reserve in peacetime and reduced to trade protection roles (hunting raiders) in the Atlantic and/or Indian Oceans with a reduced complement of 24 aircraft. At least part of that reduction is due to those aircraft being larger.