I do not really have any significant examples of changed opinion, since I normally get it right the first time.
While this does not really count as a changed opinion, I was however reluctant to try KOTOR when it came out, because I thought it was going to be some PT stuff. I did eventually gave it a try a couple of months after the release and it was (still is) one of the best Star Wars things ever made.
Years later, I was reluctant to try SWTOR too, but for a different reason (because I hate MMORPG since it takes one out of the story). I gave it a try at the release, and again, it was one of the best Star Wars things ever made (at least the original game content, most of the expansions were kinda shit). And it turned out that MMORPG elements could be ignored and the game could be played as a single-player with full story immersion.