Add Toggled Air Defense to Ships

Give your battleship AD/AA status on-the-fly.

Others have created Air-Defense (AD) ships as separate units. However, you can use the mirror-unit method to create a toggled AD status for any ship including battleships and aircraft carriers. Simply create, say, an Iowa-class battleship as an AD class and then assign a "regular" capital-ship-class Iowa as transport to the base AD unit. You similarly can add toggled anti-aircraft (AA) ability to any ship. Use a modified icon to identify status at a glance. Perhaps you might give toggled AD ability to modern battleships and CLAA cruisers and give AA ability to dual-purpose-gun destroyers.*

CL54 USS San Juan
an AA cruiser with dual-purpose 5-inch guns
instead of a cruiser's usual 6- or 8-inch guns:

The result is not purchasable but ships are not purchasable in PG/AG anyway. Depending on your combination of move and fire activities, you might end a turn in an undesirable status or have to forgo a move but overall this system provides more flexibility and more realism.

*Whether you use this method or not, remember that the United States, Japan, and Britain used dual-purpose 5-inch guns so check the AD/AA firepower compared to other nations' firepower.