Everyone's pretty much answered this as best it can be answered, but I'll drop my 2 cents anyway.

It was the easiest way for them to bring in a new character without technically killing an older one. I mean, part of the reveal is that the real Yda had died many years prior off screen. But the Lyse transmutation of the current "Yda" that happened on screen was one of the first times they've replaced a character without killing/seeming to kill a character.

If you weren't around or don't remember, when the Stormblood trailer first came out, it showed the WoL on Monk sparring with a woman in a red dress. Of course, we know now this was Lyse in Yda's old ornamental Ala Mhigan outfit. But back then, theories flew everywhere about who this was, and a lot of people settled on that it was Yda but somehow her Archon tats were gone. People also were cocksure that she wasn't just a Monk but was a Dancer.

It's very much what Moose put forward. They wanted to tell a story revolving around a young, pretty Ala Mhigan native girl who was old enough to be touched by the Garlean occupation but too young to have done anything about it. To be our native stranger who understands all the customs and traditions but has never lived by them. As the narrative setup for reclaiming Ala Mhigo, she also has to reclaim her heritage.

Can't really do that with a 40 something MILF who has more experience than almost the entire cast and all of the Ala Mhigan Resistance to boot. Of course, I wouldn't have hated that. *pushes index fingers together*

Of course, all narrative rationale aside, you should know these XIV devs by now. They've made it their mission in life to subvert expectations, and even when they actually fail to do that, they still like to act like they did.

Cause I mean, the majority of folks got it right. The girl in the trailer was, "Yda." The one we know. But XIV dev team got to play into their favorite fetish when they revealed she was actually Lyse.

Yes, that's right, our lead XIV dev is something of a loveable dumb ass.