User blog comment:Miskos3/Fairy Tail Chapter 419: The Message of Fire, Review/@comment-7070784-20150207115857/@comment-7418318-20150207131749

As B214 already said, the Dragons produced antibodies inside their childrens' bodies so that they wouldn't turn into Dragons even after the parents were out of their bodies.

Unless something purges those antibodies or the Dragon parents were mistaken about the effectiveness of those antibodies, the Dragon Slayer will never turn into Dragons.