They didn't want to do cute forever because they worried fans would get bored, so 2019 was a good time to do so, since they were pretty much in their 20s (except for ChaeTzu) but Chae's birthday was pretty much the next day.
I think this a great way to change concepts, them all agreeing. And their bridge was so smooth. I don't think that 2020 cbs were completely mature, but nothing like before 2019.
Nayeon, Jeongyeon, Jihyo and Chaeyoung all talked about it but I think Jihyo's mainly answered the question and a lot of Chaeyoung's.
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Here's my source: https://www.koreaboo.com/stori…on-changed-concept-fancy/