I never change my mind–thanks to the teaser trailers, I always appreciate every Pixar film since first viewing.
I certainly appreciate The Incredibles more than I use to.
I certainly hope to change my mind about Finding Dory. Right now, my opinion of it is not very high.
I am extremely interested in learning more about Dory’s family.
Ok guys, you’re never going to believe this, but actually, I do have a Pixar movie that I changed my mind about way after seeing it the first time… and that is… drum roll… WALL•E!!!
Here’s the story. When I first saw it in theaters, I just thought it was ok, and that it reminded me a bit of Star Wars and Doctor Who (two series I was seriously crazy over back in 2006-2008), and didn’t think more of it. My brother got the video game for the Xbox, and I got to know the minor characters, but I just thought it was nice. I had also planned to film a parody with costumes, but never did.
However, one day, when I was sitting in a restaurant, I was thinking of writing up a story, and the plotline and backstory of WALL•E suddenly hit me for inspiration (it was actually a story based on a Doctor Who episode). I had also given the game a chance, found myself saving images from the movie, begged my mom to buy the 3-Disc Special Edition when it was released, and inspiration continued to hit me (Star Vacation was born after making up parody skits with earlier characters I’ve created, and having watched Camp Rock and Sonny With a Chance), which then led to the accidental, yet fascinating, discovery of humanized fanart (yes, I wasn’t the only one!!!), and finally ending up on this site! 
But yeah, that’s my “Ever Changed Your Mind About a Pixar Film” story. And it’s one that you’d least expect.