Responses
I can't find any article about it despite my efforts but I think it's because it changed Publishers.
It used be published by a "Rain Forest" which seems to be a subsiadiary of Seoul Media Comic, Inc., or somth.
But now, Ridibooks handle its publishing and serialization.
That's what I can infer from my research but dunno if it's correct or not.

Do anybody know the story behind why the art style change?