I know Riot probably has a design "locked down" since she's been officially announced
Actually, at this point, where they only announce things, the design isn't locked (neither for the gameplay, nor for the visuals and narrative), but rather 'just' the direction.
Basically, they know what they generally want to do with the champion, and they're currently iterating on these idea/that direction (although, keep in mind that Team Alpha still has Galio on their plate, so they aren't working on Evelynn full-time yet).
-_-
Also, if we wanted to use the correct terminology:
Up until now, they were in the Ideation Phase for Evelynn and Urgot, then they decided on a modern direction for the champions that sounds good, makes sense and has the teams are actually excited. Then Reav3 makes the new State of Champion Update post.
Only then, they enter Pre-Production, in which they iterate on the direction they decided on.
By the end of Pre-Production, the more important details (a final concept for the updated base skin, most/all abilities, most of the updated bio) will usually be locked.
Afterwards, they enter the Production Phase, with which the focus will shift onto actually bringing the champions to life (models, updated skins, tweaking and iteration on specific mechanics and number in the new kit, writing the story, making the splash, finalizing voiceover, new icons, new splashes, animating the login screen and creating the login theme and more).
