User blog comment:IamJakuhoRaikoben/Chapter 345: Someone's Voice, Review./@comment-5871436-20130729210620/@comment-7418318-20130729212110

Mirajane can also use transformation magic to appear in pretty much any way she pleases.

Since she doesn't intent to actually fight there's not really any reason to do an actual take-over, so she might have just used "regular" transformation to appear in that shape.