Kingdom Hearts HD 2.5 ReMIX doesn't come out until December 2nd of this year, but in the meantime you can get a better look at the upgraded visuals (and some new costumes to boot) in the newly released SD/HD Comparison trailer below. The trailer focuses on the trio's time in Christmas land, but you can peep some additional screens in the gallery below that showcase some of the other worlds, such as Tron, Hercules, Pirates of the Caribbean, and more.
With the release being closer and closer, it might be time to replay the first one since I remember the intricacies of that plot anymore. I can't help but get excited for the 2nd ReMix with Kingdom Hearts 3 within sight.
