For me, the logic would only make sense as a visual and vocal appeal to Japanese fans who want LINO LEIA+Emiko (as they warm up to her) to reflect a new Wakana+Keiko combo. I noticed that Keiko and LINO LEIA were often taking pics and hanging out together back in the day.
Looking back, Wakana was the main soprano during her time, because she led so many songs, and Kaori was the alternative soprano lead with a more rock/belt-y edge. Interesting though, that Wakana's default standing position in FJ was audience left, and in Kalafina was audience right, but never centre, while Keiko was always in the centre.
LINO LEIA isn't yet able to properly lead so many songs (including many sung by Wakana) and so the main singing duties are shared between Joelle and Kaori.