User blog comment:Woohoot/Things that totally could happen, but won't cause... Manga.../@comment-10543621-20150524125222/@comment-25926617-20150524145818

In the Arlong Park arc Genzo and Nojiko streched Luffy's neck when he was underwater so while what you said makes sense it apparently doesn't.

Also i don't think that Haki actually changes him back to normal i think it just hurts him somehow. It doesn't get rid of powers it just bypasses them. Similar to how a logia can be in there fruit form but then you can grab them with Haki and that part doesn't change back.