The protagonist, is the main actor, no matter what his character is--good or bad. Normally, it centralized the story with the protagonist.
On the other hand, the antagonist is the opposing character of the protagonist, with conflicting characteristics with the protagonist.
What if the protagonist and antagonist change roles? Is there any special term for that? Like when either the protagonist and antagonist lead the story, having different plots, and both have the part in causing the trouble. Some stories use an interrelated parallel story plot that helps one another develop each plot.
So, back to the question, how do you call that instances? How if the protagonist act as the antagonist, and vise verse for the antagonist? What's your idea about this?