Tom's Modelworks (www.tomsmodelworks.com) does a square bridge conversion kit, to allow you to modify the Tamiya 1942 into ANY of the 175 ships in the class, from 1942-45.
Your choices are pretty simple...for the round bridge...
Replace the forward 20mm tubs with 40mm Mk 1 tubs and mounts, and build the rest of the ship out of the box...that's good for most of the round-bridge Fletchers through mid-1943. Add in the midship Mk1 40mm sponsons and mounts, and you've got most of the round bridge Fletchers through 1945. Add the Mk2 quad 40mm, replace the forward torpedo tubes with the MK 51 FCD platform, and you've got the 1945 version of the Emergency AA fit Fletchers.
For the square bridge, you can model the early ships by replacing the forward 20mm tubs with the Toms 40mm part. Square Bridge Fletchers never carried 20mm guns in the forward tubs into combat. You can then add the midship MK 1 or MK 2 40mm if you want late war ships.
I recommend my book 'Fletcher, Gearing and Sumner Class Destroyers in WW2' from the Warship Perspectives line...available from White Ensign Models and Hannants in the UK, or from Pacific Front Hobbies or Floating Drydock here in the states.
Tom's set gives you all the necessary parts, and photoetch, for the model...
Regards,
Jeff Herne
(some schlub who knows a little about the Fletchers)