Question, why do we see a complete change in the body change editor, but not in the story or in the arena?
Because CC for the Yuna form is purely visual and the form itself requires unlocking in-game.
You need to max out Yuna energy (all 10 orb thingies) and then you click on the body part slot to the left of the window to actually unlock it (the "(10)" buttons, iirc). From there you get to pick any of up to 3 options (each comes with different stats) or disable it, doesn't matter, once the body part is unlocked you're free to change it at no further cost anywhere anytime.
Yes, it's 10 Yuna orbs for ONE body part.
Outright impossible to get in the story mode (at least currently, it seems like killing bosses will give you a lot of Yuna EVENTUALLY) and still takes way too long to unlock in the arena mode.
From there the actual appearance depends on how much unspent Yuna you have. Most of the deformations will appear at 0 Yuna, but you only achieve the full CC look at 10 unspent Yuna.
Personally I just save-edited it and even that one took way longer than it should've.