It depends on the type of situation though Like for example, Some Antagonists can become Antiheroes, Which are Protagonists for their own gains, and then can become Antagonist all over again. Sometimes, the Antagonist has a change of heart because of something, and Becomes a Protagonist all on its own. Sometimes, Antagonist just stay Antagonists
The protagonist is defined by being the character whose Story We are following and even if the protagonist is a bad guy he is still the protagonist and his opponents are still the antagonist same goes with the antagonist
