I'm not entirely sure if it's been said or not. Currently from Wiki48, though, I'm just suspecting that it is Ano Hi no Jibun, because the members are:
Team A: Iriyama Anna, Owada Nana, Kojima Haruna, Shimazaki Haruka, Taniguchi Megu, Miyazaki Miho, Yokoyama Yui
Team K: Minegishi Minami, Mukaichi Mion, Muto Tomu
Team B: Oshima Ryoka, Kashiwagi Yuki, Kato Rena, Kizaki Yuria, Watanabe Mayu
Team 4: Okada Nana, Kojima Mako, Komiyama Haruka, Takahashi Juri, Murayama Yuiri
Would be interesting since it lists Paruru. But I will update accordingly if it is Kojiharu's grad song
Oh, if other previews come up on youtube, anyone feel free to share them!!
" ’15 has become a completely new Morning Musume。, but I want to treat the history that has been handed down to us preciously. If there was a book about Morning Musume。’s history, I think I would be happy to see my name left there."
~Oda Sakura
"Masaki loves all music made by Tsunku-san. Every single note and word stabs me right in my soul! I mean it in a good way!" (translated by Ronin - thanks!)
~Sato Masaki