User blog comment:IamJakuhoRaikoben/Chapter 515: I Am You... You Are Me, Review./@comment-24367018-20161221184843/@comment-7418318-20161221192226

That is something I've been wondering too and I've come up with a few possibilities so far:


 * Zeref tried to forcibly turn Acnologia back to being Human against Acno's will, and his resistence to it during the spell caused it to be different from Irene, who cooperated willingly.
 * Acnologia is a kind of Dragon Slayer genius who can do things even Irene can't.
 * It's a psychological thing. Due to the torment she had to go through due to her dragonification, Irene is either subconsciously blocking her ability to switch forms or is consciously choosing not to go into Dragon form.
 * When changing Acnologia, Zeref messed up. When changing Irene he had learned his lesson and can now make that change permanent.
 * Having killed so many Dragons/absorbed so many Dragon magics/having bathed in so much Dragon blood Acnologia isn't only more powerful, but also has more control over his DS abilities than Irene, thus he can do things she can't with them including switching between Human and Dragon shapes.
 * One of the Dragons that Acnologia killed had the power to shape-shift into Human form and that's how Acno gained the ability to appear Human, by absorbing that, rather than being enchanted (or disenchanted?) by Zeref.