User blog comment:ChishioKunrin/Giant Mech Fights = At Least Two Different Episodes/@comment-3257939-20140716205207/@comment-3146930-20140716214233

Aside from being captured, I also agree that the mech is either another combat robot that belongs to Mr. White and the White Fang capture it at some point, but we don't know exactly when and where this will happen in the new volume. Considering that some clips happen in the day and others at night, maybe there is a break between fights that would allow the characters to change outfits.

Another option is just that the trailer is using unfinished clips or maybe there is an explanation in the show for the outfit change. We will find out in about a week.