There was a lot to take away from the big finale of Wandavision but easily one of the most eye catching parts was the big costume reveal for Scarlet Witch going forward. It's definitely the coolest Wanda has looked in these movies so far, being a legit good representation of the source material while also being a solid update on the original design.
But for those of you thinking that this new costume, while impressive, didn't have much in the way of buildup at all throughout the story, it turns out that they've built it up since before the beginning, as in, you can clearly see it on the poster. Specifically the poster shown above, with all the televisions in front of Wanda and Vision, the one over Wanda's chest clearly features the suit Wanda ends the story owning.
So yeah, they had the new costume for the officially christened Scarlet Witch all along, but because Schaeffer and his team are a bunch of sneaky sneaky bastards, they teased at it all the way back in the very first official poster.