According to an interview made to the movie staff:
Miki: On a related note, about Black Yuna and White Yuna being separate beings……
Itou: Surprisingly, that didn't go through well.
Miki: The Black and White Yuna. Both of them are separate existences. The White Yuna was that white ghost who made her request to Kirito by pointing with her finger. The Black Yuna is an AR idol diva.
Itou: Until midway through through the movie, you'd expect them to be the same being, but they're actually separate…… everything was explained by Shigemura in the climax.
Miki: The Black Yuna is an AI crawler with limited emotions meant solely to drawn in players to 《OS》. Singing was her only purpose. The White Yuna was an existence born with the activation of an AI program midway. She continuously received upgrades through deep learning and was about to be reborn as Shigemura Yuuna. This Shigemura Yuuna was the yellow-haired girl in plain clothes that appeared before the professor as a phantom in the end. Oh, and she was also shown in the researcher's picture frame.
So yeah, they are separate beings, originally, the Black Yuna was used for OS as an AR-idol, later on, the White Yuna was created when the memories of the SAO survivors were stolen and she was the one supposed to become Shigemura Yuuna.
Link for a full translation of the interview along some extra lore of Aincrad here. It's part of a booklet given out to moviegoers in Japan.
