Not sure I like the "transition into darkness" in the new Fantasmic. In the new version, Mickey goes into the Magic Mirror more or less off of peer pressure as the Magic Mirror proclaims that there is "another realm" "where powers untold far greater than yours may awaken" but "only the bravest may uncover the mysteries". Ultimately, Mickey chooses to go (I guess to prove his bravery?) but Mickey doesn't need to prove anything! He's Mickey! I prefer the original plot where the villains more or less plot together to take over Mickey's dream (because as the Magic Mirror says "here in Mickey's imagination, beauty and love will always supplieth"). Seems to make more sense too when, at the end, Mickey says "you may think you're so powerful, well, this is my dream!".
Upgrade the tech (add in the new lighting, the new projection mapping), DON'T change the vocals over the princess segment, DON'T replace Pocahontas with Pirates (might be okay replacing the Pocahontas scene in general, not sure with what, but not pirates), DON'T change the plot device at the end (described above), DO upgrade the dragon...