User blog comment:Gourgeist/Shingeki no Kyojin Chapter 65 Review: Dreams and Curses/@comment-26004142-20150123171832

The description on the wiki for this manga has a possible inaccuracy. Historia went to inject herself, but she didn't penetrate her skin if you look at the picture.

The implication is that she injected herself and transform, but I have my doubts that she transformed. It could very well have been Eren transforming.

The point is, we don't know who transformed.