On Wed, 18 Feb 1998, Thor Odinson wrote:

I was led to believe [and please correct me if I am wrong] that at
some point in the manga, Ukyo is attacked by some sort of demon who
then replaces her, both body and soul.

No.

It's a common fan joke, which refers to how her characterization changes for
(as some fans see it) little reason towards the end of the manga.  It's not
literally true, though.