User blog comment:Miskos3/Fairy Tail 520: Dragon or Demon, Review/@comment-29874610-20170204151505/@comment-1359322-20170204153245

Igneel said the Dragon Seeds inside Natsu is the cause of a Dragon Slayer's transformation into a Dragon. Assuming if the Demon Seed is of the same function, i'm guessing the destruction of the seed means no more Dragon or Demon transformation.