What was the purpose of the Yda twist?

[Cleretic]

The thing I don't get is I don't feel the character's personality changed between being Yda and admitting she was Lyse. What you're saying would make more sense if she was a serious leader who'd been pretending to be ditzy because she was impersonating her sister, but I read it in reverse that the Sharlyan scholar was acting ditzy because she was really her younger sister the whole time. I think that lack of change in her personality was why so many objected to her becoming a world leader at the time.

I think the important part is less that the character changed (although she did), but that every element of the presentation did.

Doing Stormblood with Yda would be like trying to make a serious drama about organized crime and corruption in Ul'dah with Godbert Manderville as your central character. You're going to need to do some serius work to make sure that the presence of the character alone doesn't destroy the entire tone and ideal of the story. If you put Godbert into a scene straight out of the most tense moments of Yakuza, it just wouldn't work, because we'd all be spending the entire scene waiting for the jokes to start; Yda there without a full overhaul and review would have the same problem, it'd feel like putting Mr. Bean in Saving Private Ryan.

Overhauling to the point where she's not even Yda by name creates other problems, but at least the tone holds.

Also, Moose is right: if Papalymo was dead but Yda was still Yda, she'd be the oldest Scion by a long way, and that'd feel extremely wrong for the character journey ahead of her. She'd be older than every non-Gosetsu major character in Stormblood! She'd be learning from Hien while Hien was like ten years her junior.