Conditional Event that checks if the character's HP is equal to some value.
Yeah, thats basicly what I said. But I'm pretty sure the best way to do what he wants (what I think he wants), is put it in a Common Event set by a Parallel Processing. All he would have to do is turn on a Switch in the very beginning of the Game or what have you, to make the Common Event check the Hp vaule of the Hp Variable. He can use this for the Swaping thing.
I have a problem here, I'm a bit confused. At first it seems like you want to give a Game Over to the Player when he triggers an Event, say a Trap Event. But then you talk about them walking around and you want to be able to Swap the main Sprite for the Sprite of the next person in your Party. Which is fine, I understand why you're confused. It's just, if you give a game over...then it's a game over, ergo you can't do the swaping thing. But if you want to have them, when knocked out, swaping for who ever is next. Then you need that Common Event I spoke of before and the Conditional Branchs Reading the Hp Variable of every actor, then changing sprite Graphics.
If there is something I'm missing, please tell me. As always I hope I've helped. Also, I like the Idea of the Swaping Party members for the Healthy one, nice work.