weren't the original Metroid games (as well as the prime games) and Star Wars movies team efforts.
Also,
http://www.pokemon-movie.jp/comment/index.php
This interview might have the answers. We just need to translate it.
My Japanese is bad, but it seems as though they changed the voice to fit Mewtwo's role in the movie. They tout that it focuses on the friendship of Ash/Mewtwo/Genesect to show the relationship of humans and Pokemon, which makes sense because in what I've read of the manga Ash was trying to befriend one of the Genesect, I think it's saying something like the change was to reflect that understanding(or misunderstanding) with Mewtwo.
Someone on tumblr translated the interview:
Yuyama Kunihiko-Kantoku x Takashima Reiko Interview
Takashima Reiko has been announced to be performing Mewtwo in “Extreme Speed Genesect Mewtwo Awakens”!
After the recording of the most recent trailer, Yuyama Kantoku and Takashima-san did an interview together!
● What are your thoughts on performing the voice of Mewtwo for the first time?
Takashima:
Well, I was extremely happy! Afterall, its that Mewtwo!?
I haven’t necessarily had a lot of experience as an anime voice actress, but I still wanted to be considered for it.
I received a lot of coaching and orders from everyone because I wanted to expand my potential, so I was fully motivated to do it.
But still, since Mewtwo is such a great pokemon, I really wanted to do my best.
I think kids are really anticipating watching this movie, so theres a lot of pressure to live up to their expectations, but I want to draw out a lot of Mewtwo’s various potentials.
● About Mewtwo’s appearances since the first movie
Yuyama:
Since Mewtwo was the pokemon to appear in the first movie, all the staff, including myself, have an emotional attachment to this pokemon.
Mewtwo seems to overlook all the other pokemon, so theres an image as if its in a much higher place.
In this work, I wanted Mewtwo to create a new image, so this production advances it.
● About Takashima performing as Mewtwo
Yuyama:
Although its only a preliminary announcement today, we have done quite a bit within such a short amount of time.
Since I’ve listened various Mewtwo voices, I’ve really enjoyed the post-synchronization of this version.
● Is there anything you would like to highlight in this movie?
Takashima:
In each Pokemon movie, there have been a huge amount of messages compiled together.
This time, with Ash, Mewtwo, and Genesect, I think you get to see several deep friendships and bonds that humans and pokemon exchange between one another. Also, with fellow pokemon helping each other out and people working together, I think its a fun, interesting, and exciting movie to enjoy.
● Are there any points worth noting from the Director?
Yuyama:
I think the biggest theme of this pokemon [movie] is communication.
Pokemon and Ash [and his friends] understand things differently [from one another].
The biggest part is when they meet Mewtwo.
Mewtwo accepts these different qualities as they are, and that is a big thing for communication.
I will be so happy when everyone is able to enjoy how Ash reacts to meeting Mewtwo and Genesect together at the movie theater.
Look forward to the theatrical release on July 13th (Saturday)!
It' seems to be the same mewtwo.
