Do a common event, and make it a parallel process. Use a switch to turn it on, and have a parallel process on the first map that turns the switch on.
For the actual event, make a conditional branch.
Have the conditional Branch state that if Actor Aluxes has Iron Armor equipped.
Conditional Branch is on the 1st page. Actor has ____ Equipped is on the 2nd page of the conditional branch.
For the event satisfied part, have it change actor graphic.
Change Actor Graphic is on the 3rd page. You can change both the Sprite and Battler from here.