Warships being expensive, it was allways tempting to rebuilt/convert warships for new roles. What of the many programs was more useful? (I am leaving aside modernizations, since they retain the ships mission, and focusing on rebuilts that change the ships mission, this is of course a debatable criterium)
I sugest a top three:
1. Lexington/Saratoga from BC to CV.
Had the advantage of being converted while still building, their BC features (size and speed) jump started the large CV concept.
2. V&W escort convertions in WW2
Gave Britain a large number of capable escorts when they were most needed
2. Type 15 Frigate
Turned obsolete WW2 Destroyers in capable ASW escorts. gave the RN the experience to develop the excelent Type 12 Frigate series.
Sugestions, Comments?