The Kanzenban Edition, is the original Japanese cut, with the inclusion of an extra ten minute prologue, and a variety of redrawn and re-animated scenes, including new CGI effects. This version, addressed as 'The Kanzenban' aired on July 8th 1999 and became the permanent replacement of the theatrical cut in physical releases and airings in Japan (the exact same issue that the Star Wars Special Editions did to the theatrical release).
Compared to 'Pokemon The First Movie', the English dub - this is an entirely different film from start to finish. The ten minute prologue was cut out, the soundtrack was almost completely replaced and roughly 90% of the dialogue was rewritten to tell a different story, and sadly these dialogue changes were done intentionally, in an effort to change certain plot points and rewrite certain characters, such as Mewtwo, a character with a plethora of depth surrounding existentialism in the Kanzenban, and turning him into a one note villain.
The sheer amount of liberties 4kids took on their dubbing feels criminal. The existence of the Kanzenban and 'The First Movie' is practically the same thing as both 'Justice League' and 'Zack Snyder's Justice League' existing alongside one another. In both, there is a version which has been heavily edited so much it has created an entirely different movie,
If you have any interest in reading more about how these films are so drastically different, I can't stress enough how good this comparison is. Dogasu covers absolutely everything and shows just how insanely different both versions are:
https://dogasu.bulbagarden.net/comparisons/movies/mov01.html
If you'd like to read more about The Kanzenban release, Dogasu also covers this in detail:
https://dogasu.bulbagarden.net/movies/1998_pm_mewtwo_strikes_back/pm_mewtwo_strikes_back_kanzenban.html
