Since I got the Junior Novelization, these are ALL the things different and completely missing in the movie.

First, a particular thing I need to tell you all: I had a feeling the storybook pictures were somehow connected to the movie's original script, and IN FACT, they are. The Junior Novelization tells the story as the original script for the movie told it.

Character and places' names:
-Liam comes for the Kingdom of Stuffingsburg, and Duke Trentino is his uncle.
-The lady in the long purple dress is called Lady Hamilton, and knows Tori, Meredith and Trevi since quite a lot of years.
-The one who painted that portrait of Duchess Amelia is called Monsieur Pierre.

All the differences between movie and book:
-Limburger calls Keira towards the end, when Crider's getting away with the plant, and tells her that if she doesn't go back to the concert in 5 minutes, she can kiss her career goodbye. Keira throws the phone away and... well, loses her deal with the record company.
-The whole 'Diamond Gardenia' problem is quite more intricate than in the movie. The day Tori and Keira decide to switch places, Crider and Rupert go back to the palace and while Crider distracts Duchess Amelia, Rupert steals the diamonds from the plant. That night, Rupert suggests Crider to grab the whole plant and he agrees with him that they should steal the entire plant.
-Keira wears a different dress at the tea party. It's a long purple dress with a blue skirt. The boots and the hairstyle, though, are the same.
-Tori doesn't transform Crider's hairstyle in the end. The only thing that changes is the dress.
-Tori decides to go do the concert at Keira's place both 'cause Limburger can destroy her career and because she sees Emily and Charlotte in the crowd.
-When Crider calls Rupert for that gum on his pants (sorry, it's not the jacket in the book), Rupert is on a ladder and falls when he tries to get down and help his boss with the gum.
-Daniel talks more.
-Keira insults Duke Trentino, remarking him the fact that in Stuffingsburg women aren't so welcome. He gets really angry and it's Liam's job to calm him down.
-The first way Tori and Keira think of for stopping Crider is holding with their belts to the light cables and reach Crider that is on the carriage, to get both the plant and the diamonds.
-Keira is the first one to find out the diamonds have been stolen, King Frederic tells her that.
-The fairies make the necklaces for Tori and Keira, and the two girls plant the entire necklaces to make the Gardenia grow again at the end.
-Keira actually TRIES to write a song before singing I Wish I Had Her Life.

Tori and Keira's magic spells:
Those we already know:
-Royal Reception Number Nine (Tori's loose hair with the crown)
-Formal Gown Number Nine (Tori's pink party dress)
Those that aren't mentioned:
-Pop Whirlwind Number Five (Keira's purple hair with the silver headband, in the movie it's 'Keira Pop Hair Number Five')
-Popstar Daytime Number Sixty-One (Keira's purple long dress)
-Royal Princess Number Forty-Three (Still Tori's loose hair, but without the crown)
-Ball Gown Number Fourteen (The dress Crider wears in the end, the light blue one)

Differences of the dialogues:
Too many to be mentioned here.