Tristana is pretty popular and visible in the pro scene so they wanted fresh art for Worlds. The VU team couldn't have come far enough to give cues for the splash at that point so they did a version of the old Tristana. They said they'll just repaint parts of the new splash now so it matches her VU.
The new design is a bit more rural with "lighter" military garb instead of the metal boots and gloves. I think it's a great fit. Bare toesies with claws to give her more animal-features, as with her ears and eyes. She looks less like a smurf now! Midriff-wear for the summer, I guess! (It's a common way to separate the shapes and "fields" of colour, creating more "interest" for the eye. Visual design often takes liberties with practicality for the sake of aesthetics. Artist can get away with bare stomach's for female characters.)