Use the combatant's Conditional Prefabs for that, e.g. using an (object) variable condition to control when to change it, e.g. setting the change up as an ability for the enemy to use, animating and changing the prefab (by changing the object variable) in the battle events.
You can also change the battle spot's position in the battle event, it's available as a Waypoint object instead of Actor.
Animation wise, since it's still the same combatant using the same animation setup, it should match the original's animator controller (depending on your ORK animation setup), e.g. parameter or state names should be the same to play the same animations (even if they're different).
Another alternative would be having both versions on the same prefab, enabling/disabling them as needed (can also be done via events using an Activate Object node).