I like How it is switches in this part of the arc, I think it happened around the time that Camie was captured or when Luffy hit the Ruler Prince guy.
Wither way I think it fits, normally One Piece and Luffy in general are very goofy and funny looking, were as now he looks very serious like its all hitting home how much he's endangered his friends.
Any one know why it changed? I've seen it happen before, any one know why? Do the animators take turns, or do they do it on purpose?