Sometimes Angels, and Demons take a supernatural biological form beyond their natural non-corporeal metaphysical state. When this happens they are living dead entities possessing paranormal DNA. Paranormal living dead DNA has the ability to alter itself based on the state of mind, and soul of a person. An Angel in such a physical form could be seduced by evil, and his DNA would fall from grace thereby becoming Demonic in nature. A Demon could also see the light, and rise to grace. His supernatural DNA would gradually transform into Angelic DNA.
This happens because Angels, and Demons are related beings. Angels existed first, and over time some turned evil forming the first Demons. The most famous case of altered supernatural DNA is the Anti-Claus, brother of Santa Claus. Both he and Santa were born of an Angel father making them Demi-Angels. Over time the Anti-Claus turned to the dark side, and his DNA came along for the wretched ride into hell. Although some hold out hope that his human half will help him see the light, and regain his Angelic birth right so he may join Santa in spreading the spirit of Christmas.