It could be possible, but normally the antagonist is what challenges the protagonist, and is a different person altogether. But, it could be possible in two ways I can think of. Possibility 1) in a 'man vs. self' conflict where the protagonist is battling their inner self (but I don't think that their inner self would count as a villain.) Possibility 2) The original protagonist-hero dies off and the character who was originally the main antagonist takes their place as the protagonist. (Sorry I just wrote an essay)