User blog comment:IamJakuhoRaikoben/Chapter 415: And It Shall Become Your Will To Live, Review./@comment-7418318-20150113104632/@comment-4829046-20150113105423

To add to that theory, maybe Lucy can learn to do something similar that can also undo Dragonification. That way, it would be okay if Natsu becomes a Dragon or Sub-Dragon in the final battle since it could just get reversed before it becomes permanent. Sorta like with Let.