User blog comment:Miskos3/Fairy Tail Chapter 419: The Message of Fire, Review/@comment-7070784-20150207115857/@comment-7418318-20150207134256

It's not completely impossible, but it'll require a more involved reason than just "The Dragons are gone now." ^^

A relatively easy way would be for Zeref to draw those antibodies from their bodies or neutralize them in some way. He could use them to create a new kind of Demon, remove them to force the DS into becoming Dragons or some other mysterious reason.

There could also by myriad other ways for them to become more Dragon-like if Hiro wants it, but it won't just happen by itself.