Why: DMCA takedown request is a very specific process and it requests immediate removal of content. Poster can then dispute it to get it restored. (http://brainz.org/dmca-takedown-101/) As far as I understand your post, that's not what you submitted since modifying the content / replacing the author is not one of the DMCA outcomes.

Possible: "Asking SE to reown/remove plagiarised content"

Why reowning is a bad idea? Because you didn't create that post and the post has effects inside of SE system (reputation, comment addressing, etc.). Nothing of course stops you from requesting a takedown and posting the same content again from your own account. Request to reown some already posted content seems a bit dodgy to me.