Wait, how did Replica-chan survive? What's with Sunao's complete personality 180? Who pushed her and why?
The personality change definitely started since the 1 month disappearance of Nao.
Sunao summoned her again and asked her if she was dating Aki, which Nao at the time said no.
It seems during the 1 month Sunao didn't summon her she realized the effects Nao had.
Nao was her own person, had her own conversations, spent time and made memories with others especially with Aki (at the time they still didn't know Aki was also a replica)
When Sunao was finally force to go to school for 1 months, she probably started to realize this the most with the conversations that probably came that she couldn't follow especially with the club.
And she was shown to have looked and felt guilty for what she did.
Sunao had basically stripped someone of 1 whole month of their life as if she was nothing.
Ever since then, Sunao has only ever been some trying to be more considerate and kind.
So it's not a sudden 180, it was a gradual change, but since we the readers see this story from the pov of Nao we didn't exactly get to see all that.