User blog comment:Miskos3/Fairy Tail 520: Dragon or Demon, Review/@comment-4933386-20170205092919/@comment-7418318-20170206105524

You could say that the seed is the catalyst that causes the changes in the body, but once the change is done, it's permanent and won't go away even if the seed is destroyed.

And juding from what just happened to Natsu it would appear that getting rid of the seed in time will prevent any changes from happening in the first place.

But there is one thing I've been wondering about. We just learned that the two seeds were interfering with each other. So does that mean that the Dragon Seed prevented the Demon Seed from working and if Natsu hadn't become a DS, he would have irreversibly turned into a Demon long ago?

Of course that's just food for thought with little bearing on the story, since it's just a what-if, but still an interesting thought.