How did Toshinou Kyouko's personality change?
I believe that because Kyouko was being bullied, she became shy and wary of people and her true personality was being suppressed.



Once Akari came to stop the bullying, Kyouko would have been able to have more confidence in herself, letting her express herself more, becoming the Kyouko we know and love today.

As for an official statement, I can't seem to find one on the author's Twitter account, or from googling - but I'm fairly certain it would be a similar answer to the one above.