It would basically replace 2 destroyers.
Uhhh, no.... A single Atlanta has 6 twin guns, equaling a total of 12 barrels. That would be three destroyers, going on the premise that 4 turrets are available on a Fletcher. In addition, and I'm not sure they modeled it in AH or not, but the Fletcher used a 5"/25 caliber gun, while the Gearings and Sumners, and all larger class ships with twin mounts (Essex, Pittsburgh Class CAs, Atlanta Class CLAA, Iowa Class, SoDak, Washington class, etc) all used 5"/38 caliber weapons.
What you're asking for, a radar control ship, didn't happen in WW2. A WW2 ship was USED as the first radar control ship, the USS Saipan, a CVL, but that was post war.
I have no problem with eliminating auto-puffy and adding an Atlanta Class CLAA to compensate, or even upgrading the Fletcher-esque destroyers we have now to Sumners. In fact, I'm all for it. But sinking a ship and having that one ship reduce the effectiveness of the AA overall is not a great idea.