While we still have no word on Diana's release date, her skin, Dark Valkyrie Diana, has been updated in the latest PBE build. It now has a much more vivid color palette, featuring green and purple instead of just dark purple, and a new splash art to match.
Here is a comparison between the old model and the updated model.
Battlecast Cho'Gath also got a few updates, but nothing extensive. He was renamed to Battlecast Prime Cho'Gath and has had some slight particle / sound tweaks.
Just speculating, perhaps the name change is to differentiate this higher quality (and priced?) Battlecast skin with the Battlecast Urgot, Battlecast Xerath, and, possibly, the future Battlecast Skarner skin.