Celestia was genuinely afraid of Discord, right? But come Keep Calm and Flutter On, she isn't afraid of him at all. What caused such a drastic change?

7525334 I never got that impression, especially not based on her interactions with him in both "The Return of Harmony" and "Princess Twilight Sparkle."

7525334
Probably the animation, the writing, and the directions given to her voice actor.

7525334
Not so much a drastic change as more like the result of Season 3 not being able to justify the change better.

7525334
My guess is she felt confident that he was contained thanks to Twilight already beating him once. Nothing tones down intimidation like a losing streak.

7525334
She needed to lose her fear in order to bring him back in to appeal to the fans. He returned and was “reformed” because the writers deemed him too popular with the fandom to be left out.

We all saw how well that went.