First - it's just my personal interpretation.
Butterflies are some kind of personal symbol for Kuu, she uses them a lot,the only tattoo she has is a butterfly e.g.
Also Butterfly was the song which got her big in 2005. real emotion/1000 no kotoba and Cutie Honey before were hits as well, but Butterfly was the start of her peak; it was song of the year 2005, she got her first award ever for it.
And it was her first hit song she did "on her own", not being chosen by a producer or being asked to do it like for Cutie Honey, a cover song.
Kuu says Can We Go Back is about her weakness in the time of the scandal, about the fear to go on.
Together with the lines after the butterflies, it makes more sense:
"The butterflies will return right now
We’re allowed to throw everything away, right?
Giving up on what we used to be is easy
This isn’t how it’s supposed to be"
So for me she is refering to the time of her first big success, where she found herself and the confidence to be like she wants to be. People started to accept her and her image. She called the years before her "winter years". After the scandal 2008 and 2009 she didn't feel well again, lost, like in those years before her ero-kakkoi image, before she knew what she wanted to portray with her singing career.
Can We Go Back was the point for her to change her attitude again.
It's also in the first verse of the song:
"When I try to think things over
I feel like I’m lost in a maze
It’s like if I can’t see the exit anymore
All this is killing me"
So the butterflies returning in the beginning are the strength and confidence returning to her which she first had with her ero-kakkoi success imo.
She also used the butterfly hanging upside down for the tour live of Can We Go Back, probably as a symbol to get her life in the right direction again, the butterfly being herself.
All those quotes of her I'm using (and still much more insight) can be found in Koda Reki, tenshi_no_hane translated it
here.
I guess my interpretation probably makes more sense reading it.
I still have the feeling there's so much more to say to make my point clear, haha.