Actually, I think it'd be pretty cool if the past members were to appear in Kudo's grad because imagine this scenario:
It's the encore, Duu sings her grad song and then the messages from the current members start rolling out. Everyone's crying, things get emotional and then it's Duu's time to leave the stage - however, she doesn't go backstage or something like that, yet she goes forward, across the stage (I'm already assuming we're in some arena rn btw :'D) to the other side where all the graduated OGs are there welcoming her.
I think it would be kind of cool if this happened but here's the catch....they're all wearing their graduation outfits!
Now imagine this for the 20th anniversary concert...
The music for Brand New Morning starts playing and we can see shadows of the girls coming onto stage. Then the song starts and the lights come on, revealing the OG members!! They start doing the dance and when it comes time to sing the current members pop out singing "We are brand new morning" yada yada and a mix between past and present performs the song.