From: A Lion uses all its might to catch a Rabbit
The metamorphosis of the Japanese CS into CVLs is a regular issue raised. It has been answered many times before. Krigan on 28 January 2012 over in the War Room gave a good summary. Symon, on innumerable occasions (see in particular this thread http://www.matrixgames.com/forums/tm.asp?m=2834896&mpage=1&key=ship%2Cconversion) has explained the difference between "conversion" and "upgrade" which is a fundamental concept relevant to this metamorphosis. Both Symon and Don Bowen have on innumerable occasions explained why the National Base (= Tokyo) was chosen to prevent a cheat which became obvious in testing AE. This cheat involved undertaking upgrades of non capital ships to capital ships in most inappropriate locations.
Even though it is probably a futile attempt, here is the consolidated list of factors which Japanese players need to take into account if they want to change the Chitose, Chiyoda, Mizuho and Nisshin from non capital CS ships into capital CVLs.
1. People regularly use the terms "upgrade" and "conversion" interchangeably. This is not correct and leads to confusion. The terms have quite separate meanings. Most ship classes have subsequent "upgrade" paths. A few ship classes also have a "conversion" path option. However, these two paths are mutually exclusive. The Mizuho CS class has further upgrades, as a CS, in April 1943 and June 1944. However, if the player elects to change it into a CVL in December 1942, then only the CVL upgrade of July 1944 is available.
2. Ships can "upgrade" along the historical, real world paths which occurred to their ship class. A ship which historically was sunk but is not sunk in the game can be "upgraded" to the same standard which it's sister ships in the same class were historically "upgraded". In some instances, where all ships in the class were sunk before any historical "upgrades" occurred, the scenario designer has provided a realistic upgrade option which is based on what similar ship classes experienced. The various Mizuho upgrades referred to in point 1 above is a case in point of realistic upgrades made available by the scenario designer.
3. Unlike "upgrades", ship "conversions" are ahistorical options, albeit intended to be as realistic as possible. To put this into context, both the Mizuho and Nisshin have the option to "convert" in December 1942 into CVLs but historically neither did become a CVL because they were sunk before the "conversion" could occur. Nonetheless, as the Japanese ship building response to Midway envisaged their possible conversion, so the player is provided with the option. On the other hand, both the Chitose and Chiyoda have the historical option in November 1942 to "upgrade" to CVLs.
4. What causes players great confusion is that whilst normally a ship "upgrade" can occur at any port which meets the specific upgrading conditions, this does not appear to apply to the Chitose and Chiyoda November 1942 "upgrades". The reason why this is so is due to the previously alluded cheat. Accordingly, because the Chitose and Chiyoda becoming CVLs constitutes an "upgrade" from a non capital ship class to a capital ship class, the "upgrade" must occur at the National Base which is Tokyo. Contrast this with the Mizuho and Nisshin who undertake only a "conversion" in changing from a non capital ship class to a capital ship class and therefore the need to be in Tokyo does not apply.
5. The conditions for the change are therefore as follows.
(a) Chitose and Chiyoda
must be disbanded in Tokyo
the ship conversion toggle must be set to "yes". Note that whilst it is technically an "upgrade" in code terms, it is the "conversion" toggle which is used to shift the ship into the capital ship pathway
Tokyo must have a "Repair Shipyard" sized 50. In some scenarios the Tokyo Repair Shipyard starts at size 10 only and must be increased in size to level 50 before November 1942.
the Tokyo Naval Shipyard and Merchant Shipyard do not substitute for the Tokyo Repair Shipyard. Only the Tokyo Repair Shipyard counts
the option becomes available on 1 November 1942
(b) Mizuho and Nisshin
can be disbanded in any port with a suitably sized Repair Shipyard
the ship conversion toggle must be set to "yes"
the Repair Shipyard must be a level 25 size. Again, do not substitute either a Naval Shipyard or Merchant Shipyard.
the option becomes available on 1 December 1942
6. Effects of the change
all four ships will be offline for 300 days
the four new CVLs have a 30 aircraft carrying capacity
if the original organic floatplane air units are left on board the Chitose and Chiyoda at the moment the "upgrade" occurs, they automatically convert into a 21 plane carrier fighter unit and a 9 carrier capable torpedo bomber unit on each CVL
as CVLs both the Chitose and Chiyoda have a maximum of 27 torpedo sorties
the original organic floatplane air units, if left on the Mizuho and Nisshin at the moment of their "conversion", do not convert into carrier fighters and torpedo bombers but remain as floatplanes
As CVLs, neither the Mizuho nor the Nisshin have any torpedo sorties
in some scenarios, the Mizuho gain san extra 2 knots in speed as a CVL
7. Finally, the Japanese player needs to carefully evaluate whether the change into capital ships is worth the effort.