Poppy's kit is impossible to balance currently, so I can see why they're reworking her. And they're keeping plenty of her old kit in while adding more iconic abilities.
Sion's old kit was boring, non-interactive, and didn't fit any sort of theme. Plus, his character was a joke. Just clumsy looking ogre-dude that quotes Arnold Schwarzenegger. The full rework was necessary and made an entirely new champion out of him.
Karma is the odd-one out... Her old kit was unique and had a very clear theme to it. Riot scrapped her almost entirely because she wasn't a popular champion, she had invisible power, and they felt her W was too confusing. As an Old and New Karma main, I'm still salty they didn't preserve her AP bruiser yin-yang playstyle at all. They tossed her out and made a much more generic mage-support out of her.
I'm hoping they'll take good care of Taric when they complete his kit. He's not in too bad a spot right now, but he lacks any real play-making potential. I'm not trying to get my hopes up but as long as they don't pull a full-Karma on us it should be fine.