The more I think on it, the more I like the Cathartes method; a soft switch.
That way, we will not need to put upgrade ships in additional slots, or worry about whether one kind of cammo pattern or another is appropriate for 1942, 43, 44, etc..., or have to create a hybrid art set.
Let's say every Tennessee will have bm #0180, but the soft switch replaces the PH stock grafik with a 1943 Tennessee in 1943. Similarly, for cammo people, they can create a Measure 41 folder, a Measure 42 folder, a Western Approaches folder, a dazzle folder, yadda yadda. Since changing art does not effect a game in progress, this seems to be an ideal solution. A player can have a set of subfolders for 1942, 43, 44, 45 holding only those ships whose appearance changed in that year.
No additional "upgrade" bm slots are required; no arguments about how a particular ship was painted and when; you can add radar; add/delete scout planes; keep or replace the X turret in certain JP destroyers with AA; you get the idea.
In order to provide the cammo folks and the upgrade folks with opportunity, it seems that vanilla is the way to go for a basis.
Who wants to carry this flag?