Add a window that players can open in which equipment, accessories, and costumes can be placed into it. Let the window have an "equip" button and by pressing it will swap in all the items in the window.
Example:
currently equipped with "above 50% HP" Tenebrous glove, 5/5 Bloody Nightmare, and Code of Honor
- place "below 50% HP" Tenebrous glove, 5/5 Tourbillon, and Engine Cooling System into the Gear Swap window
- enter raid
- boss reaches roughly the halfway point on HP
- open Gear Swap window and click "equip"
- within a moment, the following occur simultaneously:
"above 50% HP" -> "below 50% HP" Tenebrous glove
5/5 Bloody Nightmare -> 5/5 Tourbillon Assemble
Code of Honor -> Engine Cooling System