User blog comment:IamJakuhoRaikoben/Chapter 498: Gray vs Invel, Review./@comment-26492493-20160822192816/@comment-7418318-20160822203724

Except that Natsu would turn into a Dragon eventually if Igneel hadn't spent eight or so years producing a counteragent against it.

Silver never mentioned producing any antibodies against Demonification, so unless Devil and Dragon Slayer magic works different, he just might turn into an actual Demon with the ability to switch to Human form, like Acnologia turned into an actual Dragon with the ability to switch back to Human form.

Theoretically possible anyway...