The movie is a condensed version of the manga and anime series, thus a lot of the events that happen are condensed and some characters were changed or omitted.
Schools that appear in the movie besides Seigaku and Hyotei: Fudomine, Rikkaidai, Rokkaku and Shitenhouji
The character of Sumire Ryuzaki's age was changed so that instead of being a grandmother (and Nanjiroh's junior middle school coach), she became a year younger than Nanjiroh and became a former junior middle school student with him.
In the movie, the character of Wakashi Hiyoshi from the Hyotei school was replaced by Egate Mcleod Higaki, whereas in the manga/anime, there was no Egate Mcleod Higaki, he was created as an original character for the movie. Similarly, Egate Mcleod Higaki's sister, Shioin Higaki, is also an original character that was not in the manga/anime series. Her purpose in the movie is to pretty much serve as a double role of representing the anime/manga character of Sakuno Ryuzaki and Egate's sister.
Only the 2nd Seigaku cast from The Prince of Tennis Musicals kept their respective roles for this movie, with the exception of Kotarou Yanagi, who played Ryoma in the musicals.
The role of Ryoma Echizen was re-casted so that instead of having Kotarou Yanagi reprise his role from the musicals for the movie, Kanata Hongo was casted to better portray Ryoma as Hongo, 15, was closer to the age of Ryoma (12) while Kotarou, 20 was much older.
As well, Kouji Date was the only Prince of Tennis Musical actor from the musical Hyotei cast to reprise his role as Choutarou Ohtori. Everyone else was completely re-casted.