kanata having short hair has nothing to do with anything happening in the !!-era! the uniforms are based off the illustration released alongside Eccentric Party Night switch's first studio album in the !-era: https://static.wikia.nocookie.net/ensemble-stars/images/b/bf/Switch_album_Eccentric.jpg/

i phrased in the last answer that it was from "reminiscence" perspective; i apologize if it was unclear, but it's as if the characters are "looking back"; meaning that it is made based off the eccentrics as they were in ! era (after the war) as they reflect upon those memories. the illustration was created accordingly. that is why kanata has short hair in the illustration, and why rei sings in his usual tone in the song (and not his 2nd year tone)

but unfortunately, there's no evidence that there was a reunion that actually took place back then and no evidence that they ACTUALLY ever sung or recorded such a song. if anything, they avoided performing as the five eccentrics while attending yumenosaki as to not bring up sentiments from the war.

they DID get to have a reunion however during OO, in Altered! but they performed as Altered and their associated song is Twilight Pentagram.